The macOS name mangling adds another underscore. Therefore, on macOS
the __atomic_* functions are actually ___atomic_* in libcompiler_rt.dylib.
To handle this case, prepend the asm() argument with __USER_LABEL_PREFIX__
in the same way that atomic.c does.
Reviewed By: ldionne
Differential Revision: https://reviews.llvm.org/D92833