Running a buildkernel since roughly a week ago (well, first attempt was after the 3.5 import) fails building for me:

cc  -c -x assembler-with-cpp -DLOCORE -O2 -pipe -fno-strict-aliasing  -g -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-tautological-compare -Wno-error-empty-body  -Wno-error-parentheses-equality -Wno-error-unused-function  -Wno-error-pointer-sign -Wno-error-format -Wno-error-parentheses -nostdinc  -I. -I/usr/src/sys -I/usr/src/sys/contrib/altq -I/usr/src/sys/contrib/ipfilter -I/usr/src/sys/dev/ath -I/usr/src/sys/dev/ath/ath_hal -I/usr/src/sys/contrib/dev/ath/ath_hal -I/usr/src/sys/contrib/ngatm -I/usr/src/sys/dev/twa -I/usr/src/sys/dev/cxgb -I/usr/src/sys/dev/cxgbe -I/usr/src/sys/contrib/libfdt -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h  -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse -msoft-float  -fno-asynchronous-unwind-tables -ffreestanding -fstack-protector -gdwarf-2 -Wno-error-tautological-compare -Wno-error-empty-body  -Wno-error-parentheses-equality -Wno-error-unused-function  -Wno-error-pointer-sign -Wno-error-format -Wno-error-parentheses -mno-aes -mno-avx -std=iso9899:1999 -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-tautological-compare -Wno-error-empty-body  -Wno-error-parentheses-equality -Wno-error-unused-function  -Wno-error-pointer-sign -Wno-error-format -Wno-error-parentheses  -Werror /usr/src/sys/amd64/acpica/acpi_wakecode.S
/usr/src/sys/amd64/acpica/acpi_wakecode.S:57:2: error: unexpected directive .code16
 .code16
 ^
/usr/src/sys/amd64/acpica/acpi_wakecode.S:57:9: error: .code16 not supported yet
 .code16
        ^
/usr/src/sys/amd64/acpica/acpi_wakecode.S:119:2: error: invalid instruction mnemonic 'lgdtl'
 lgdtl bootgdtdesc - wakeup_start
 ^~~~~
*** Error code 1

uname:
FreeBSD poolia 11.0-CURRENT FreeBSD 11.0-CURRENT #8 20daa3d(master): Thu Nov 27 11:42:07 AEDT 2014     root@poolia:/usr/obj/usr/src/sys/SIMPL  amd64

Kernel config differences:
nooptions       DDB            
nooptions       GDB               
nooptions       DEADLKRES         
nooptions       INVARIANTS         
nooptions       INVARIANT_SUPPORT 
nooptions       WITNESS           
nooptions       WITNESS_SKIPSPIN

device pf
device pflog
device pfsync

options         ALTQ
options         ALTQ_CBQ 
options         ALTQ_RED  
options         ALTQ_RIO  
options         ALTQ_HFSC 
options         ALTQ_PRIQ  
options         ALTQ_NOPCC  

I additionally tried the GENERIC kernel (same issue), and using clang35 from the pkg tree with no success