[macho] Allow CPUSubtype to contribute to architecture identification

Summary:
Sometimes the CPUSubtype determines the Triple::ArchType that must be used.
Add the subtype to the API's to allow targets that need this to correctly
identify the contents of the binary.

Reviewers: pete

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70345