"-Wl,-r " is a compiler switch to pass linker option "-r" for building shared objects, and this should work for all CC and GCC compilers on different Unix platforms.
This works prior to Clang 6.0.1 and FreeBSD 12-R, but failed on FreeBSD 12-R with Clang 6.0.1.

Below steps can easily reproduce the problem. Directly use ld to link a shared object works no issue for a shared object [step 3,4], but cc (Clang 6.0.1) "-Wl,-r" command failed to do so [step 5].


[1] FBSD120: cat << CLANG60_TEST_CASE > test-clang6.c
#include <stdio.h>

int anyFunc(int a, int b)
{
        printf("a = %d b = %d\n", a, b);
        return a * b;
}
CLANG60_TEST_CASE

[2] FBSD120: cc -c -o test.o -fPIC test-clang6.c

[3] FBSD120: ld -r -o test.so test.o
[4] FBSD120: ld -r -o test.so test.o -v
LLD 6.0.1 (FreeBSD 335540-1200005) (compatible with GNU linkers)

[5] FBSD120: cc -Wl,-r -o test.so test.o -v
FreeBSD clang version 6.0.1 (tags/RELEASE_601/final 335540) (based on LLVM 6.0.1
)
Target: x86_64-unknown-freebsd12.0
Thread model: posix
InstalledDir: /usr/bin
 "/usr/bin/ld" --eh-frame-hdr -dynamic-linker /libexec/ld-elf.so.1 --hash-style=
both --enable-new-dtags -o test.so /usr/lib/crt1.o /usr/lib/crti.o /usr/lib/crtb
egin.o -L/usr/lib -r test.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc -
-as-needed -lgcc_s --no-as-needed /usr/lib/crtend.o /usr/lib/crtn.o
/usr/bin/ld: error: attempted static link of dynamic object /usr/lib/libgcc_s.so
/usr/bin/ld: error: attempted static link of dynamic object /usr/lib/libgcc_s.so
cc: error: linker command failed with exit code 1 (use -v to see invocation)