[LTO][Legacy] Add new API to query Mach-O CPU (sub)type

Tools working with object files on Darwin (e.g. lipo) may need to know
properties like the CPU type and subtype of a bitcode file. The logic of
converting a triple to a Mach-O CPU_(SUB_)TYPE should be provided by
LLVM instead of relying on tools to re-implement it.

Differential Revision: https://reviews.llvm.org/D75067