Disable retref on Darwin

This is currently failing on Darwin due to a linker error.

http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64-O0-g/
http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64-O3/
http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-x86_64h-O3/

Disable it until this issue is fixed.

Error:

ld: in section __DATA,xray_instr_map reloc 0: X86_64_RELOC_SUBTRACTOR must have
r_extern=1 file
1 file changed