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

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 lib/BinaryFormat.

This also gets rid of the separate two X86AsmBackend classes.

The previous attempt was to add it to libObject, but that adds an
unnecessary dependency to libObject from all the targets.

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