[macho][NFC] Extract all CPU_(SUB_)TYPE logic to libObject

This moves all the logic of converting LLVM Triples to
MachO::CPU_(SUB_)TYPE from the specific target (Target)AsmBackend to
more convenient functions in libObject.

This also gets rid of the separate two X86AsmBackend classes.

Differential Revision: https://reviews.llvm.org/D74808