Recognize ARM64EC binaries in COFFObjectFile::getMachine.

ARM64EC/ARM64X binaries use ARM64 or AMD64 machine types, but provide
additional CHPE metadata that may be used to distinguish them from
pure ARM64/AMD64 binaries.

Reviewed By: jhenderson, MaskRay, mstorsjo
Differential Revision: https://reviews.llvm.org/D149091