This is another example place with the same sort
of compiler report as bugzilla 215600 was for.
I've switched back to using WERROR to allow
the kernel to build via xtoolchain-powerpc64-gcc .
The example context is a debug kernel build.
At least this place in the kernel also needs
an update if WERROR use is to be avoided
for xtoolchain-powerpc64-gcc.

The detailed build failure material follows.

--- stack_machdep.o ---
/usr/src/sys/powerpc/powerpc/stack_machdep.c: In function 'stack_save':
/usr/src/sys/powerpc/powerpc/stack_machdep.c:113:22: error: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Werror=frame-address]
  frame = (register_t)__builtin_frame_address(1);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
Building /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/sys/GENERIC64vtsc-DBG/mmu_phyp.o
. . .
--- stack_machdep.o ---
cc1: all warnings being treated as errors
. . .
--- stack_machdep.o ---
*** [stack_machdep.o] Error code 1

make[2]: stopped in /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/sys/GENERIC64vtsc-DBG
.ERROR_TARGET='stack_machdep.o'
.ERROR_META_FILE='/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/sys/GENERIC64vtsc-DBG/stack_machdep.o.meta'
.MAKE.LEVEL='2'
. . .

# Meta data file /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/sys/GENERIC64vtsc-DBG/stack_machdep.o.meta
CMD /usr/local/bin/powerpc64-unknown-freebsd12.0-gcc -isystem /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/include -L/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/lib -B/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/lib --sysroot=/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp -B/usr/local/powerpc64-freebsd/bin/ -c -O -pipe  -g -nostdinc  -I. -I/usr/src/sys -I/usr/src/sys/contrib/libfdt -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h  -msoft-float -fPIC -fno-omit-frame-pointer -mno-altivec -msoft-float -ffreestanding -fwrapv -fstack-protector -gdwarf-2 -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual -Wundef -Wno-pointer-sign -fformat-extensions -Wmissing-include-dirs -fdiagnostics-show-option -Wno-unknown-pragmas -Wno-error=address -Wno-error=aggressive-loop-optimizations -Wno-error=array-bounds -Wno-error=attributes -Wno-error=cast-qual -Wno-error=enum-compare -Wno-error=inline -Wno-error=maybe-uninitialized -Wno-error=overflow -Wno-error=sequence-point -Wno-error=strict-overflow -Wno-error=unused-but-set-variable -Wno-error=misleading-indentation -Wno-error=nonnull-compare -Wno-error=shift-overflow -Wno-error=tautological-compare  -v -fno-common -fms-extensions -finline-limit=15000 --param inline-unit-growth=100 --param large-function-growth=1000 -Wa,-many -mcall-aixdesc  -std=iso9899:1999 -Werror  /usr/src/sys/powerpc/powerpc/stack_machdep.c
CMD ctfconvert -L VERSION -g stack_machdep.o
CWD /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/sys/GENERIC64vtsc-DBG
TARGET stack_machdep.o
-- command output --
Using built-in specs.
COLLECT_GCC=/usr/local/bin/powerpc64-unknown-freebsd12.0-gcc
Target: powerpc64-unknown-freebsd12.0
Configured with: /usr/obj/portswork/usr/ports/devel/powerpc64-gcc/work/gcc-6.3.0/configure --target=powerpc64-unknown-freebsd12.0 --disable-nls --enable-languages=c,c++ --without-headers --with-gmp=/usr/local --with-pkgversion='FreeBSD Ports Collection for powerpc64' --with-system-zlib --with-gcc-include-dir=/usr/include/c++/v1/ --with-as=/usr/local/bin/powerpc64-freebsd-as --with-ld=/usr/local/bin/powerpc64-freebsd-ld --prefix=/usr/local --localstatedir=/var --mandir=/usr/local/man --infodir=/usr/local/info/ --build=amd64-unknown-freebsd12.0
Thread model: posix
gcc version 6.3.0 (FreeBSD Ports Collection for powerpc64) 
COLLECT_GCC_OPTIONS='-isystem' '/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/include' '-L/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/lib' '-B' '/usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/lib' '-B' '/usr/local/powerpc64-freebsd/bin/' '-c' '-O' '-pipe' '-g' '-nostdinc' '-I' '.' '-I' '/usr/src/sys' '-I' '/usr/src/sys/contrib/libfdt' '-D' '_KERNEL' '-D' 'HAVE_KERNEL_OPTION_HEADERS' '-include' 'opt_global.h' '-msoft-float' '-fPIC' '-fno-omit-frame-pointer' '-mno-altivec' '-msoft-float' '-ffreestanding' '-fwrapv' '-fstack-protector' '-gdwarf-2' '-Wall' '-Wredundant-decls' '-Wnested-externs' '-Wstrict-prototypes' '-Wmissing-prototypes' '-Wpointer-arith' '-Winline' '-Wcast-qual' '-Wundef' '-Wno-pointer-sign' '-fformat-extensions' '-Wmissing-include-dirs' '-fdiagnostics-show-option' '-Wno-unknown-pragmas' '-Wno-error=address' '-Wno-error=aggressive-loop-optimizations' '-Wno-error=array-bounds' '-Wno-error=attributes' '-Wno-error=cast-qual' '-Wno-error=enum-compare' '-Wno-error=inline' '-Wno-error=maybe-uninitialized' '-Wno-error=overflow' '-Wno-error=sequence-point' '-Wno-error=strict-overflow' '-Wno-error=unused-but-set-variable' '-Wno-error=misleading-indentation' '-Wno-error=nonnull-compare' '-Wno-error=shift-overflow' '-Wno-error=tautological-compare' '-v' '-fno-common' '-fms-extensions' '-finline-limit=15000' '--param' 'inline-unit-growth=100' '--param' 'large-function-growth=1000' '-mcall-aixdesc' '-std=c99' '-Werror'
 /usr/local/libexec/gcc/powerpc64-unknown-freebsd12.0/6.3.0/cc1 -quiet -nostdinc -v -I . -I /usr/src/sys -I /usr/src/sys/contrib/libfdt -isysroot /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp -D _KERNEL -D HAVE_KERNEL_OPTION_HEADERS -isystem /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/include -include opt_global.h /usr/src/sys/powerpc/powerpc/stack_machdep.c -quiet -dumpbase stack_machdep.c -msoft-float -mno-altivec -msoft-float -mcall-aixdesc -auxbase stack_machdep -g -gdwarf-2 -O -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual -Wundef -Wno-pointer-sign -Wmissing-include-dirs -Wno-unknown-pragmas -Wno-error=address -Wno-error=aggressive-loop-optimizations -Wno-error=array-bounds -Wno-error=attributes -Wno-error=cast-qual -Wno-error=enum-compare -Wno-error=inline -Wno-error=maybe-uninitialized -Wno-error=overflow -Wno-error=sequence-point -Wno-error=strict-overflow -Wno-error=unused-but-set-variable -Wno-error=misleading-indentation -Wno-error=nonnull-compare -Wno-error=shift-overflow -Wno-error=tautological-compare -Werror -std=c99 -version -fPIC -fno-omit-frame-pointer -ffreestanding -fwrapv -fstack-protector -fformat-extensions -fdiagnostics-show-option -fno-common -fms-extensions -finline-limit=15000 --param inline-unit-growth=100 --param large-function-growth=1000 -o - |
 /usr/local/bin/powerpc64-freebsd-as -v -I . -I /usr/src/sys -I /usr/src/sys/contrib/libfdt --traditional-format -a64 -mppc64 -many -many -o stack_machdep.o
GNU assembler version 2.28 (powerpc64-freebsd) using BFD version (GNU Binutils) 2.28
GNU C99 (FreeBSD Ports Collection for powerpc64) version 6.3.0 (powerpc64-unknown-freebsd12.0)
        compiled by GNU C version 4.2.1 Compatible FreeBSD Clang 4.0.0 (tags/RELEASE_400/final 297347), GMP version 6.1.2, MPFR version 3.1.5-p2, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
#include "..." search starts here:
#include <...> search starts here:
 .
 /usr/src/sys
 /usr/src/sys/contrib/libfdt
 /usr/obj/powerpc64vtscdbg_xtoolchain-gcc_kernel/powerpc.powerpc64/usr/src/tmp/usr/include
End of search list.
GNU C99 (FreeBSD Ports Collection for powerpc64) version 6.3.0 (powerpc64-unknown-freebsd12.0)
        compiled by GNU C version 4.2.1 Compatible FreeBSD Clang 4.0.0 (tags/RELEASE_400/final 297347), GMP version 6.1.2, MPFR version 3.1.5-p2, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 8ac087b8ad16a23ac09c5d0c7497aeae
/usr/src/sys/powerpc/powerpc/stack_machdep.c: In function 'stack_save':
/usr/src/sys/powerpc/powerpc/stack_machdep.c:113:22: error: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Werror=frame-address]
  frame = (register_t)__builtin_frame_address(1);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
*** Error code 1
. . .