Implement support for __llvm_addrsig for MachO in llvm-mc

The __llvm_addrsig section is a section that the linker needs for safe icf.
This was not yet implemented for MachO - this is the implementation.
It has been tested with a safe deduplication implementation inside lld.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D123751