[compiler-rt] Fix AArch64 out-of-line atomics visibility on mach-o

This fixes an apparent oversight in D91156, where the symbol was defined
without the leading underscore, then the visibility was later declared with it.
rdar://73364185

Differential revision: https://reviews.llvm.org/D95639

GitOrigin-RevId: 5f3c4923e4e4d5f40a64e08e407067ff551f2901
1 file changed