[clang] Add ARM64 support to armintr.h for MSVC compatibility

Summary: This fixes compiling with headers from the Windows SDK for ARM64.

Reviewers: compnerd, ruiu, mstorsjo

Reviewed By: compnerd, mstorsjo

Subscribers: mgorny, aemerson, javed.absar, kristof.beyls, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D35862

llvm-svn: 309081