[lld-macho] Add ARM64 target arch

This is an initial base commit for ARM64 target arch support. I don't represent that it complete or bug-free, but wish to put it out for review now that some basic things like branch target & load/store address relocs are working.

I can add more tests to this base commit, or add them in follow-up commits.

It is not entirely clear whether I use the "ARM64" (Apple) or "AArch64" (non-Apple) naming convention. Guidance is appreciated.

Differential Revision: https://reviews.llvm.org/D88629