Support __iso_volatile_load8 etc on aarch64-win32.

These intrinsics are used by MSVC's header files on AArch64 Windows as
well as AArch32, so we should support them for both targets. I've
factored them out of CodeGenFunction::EmitARMBuiltinExpr into separate
functions that EmitAArch64BuiltinExpr can call as well.

Reviewers: javed.absar, mstorsjo

Reviewed By: mstorsjo

Subscribers: kristof.beyls, cfe-commits

Differential Revision: https://reviews.llvm.org/D47476

llvm-svn: 333513