Headers: Add ARM support to intrin.h for MSVC compatibility

This fixes compiling with headers from the Windows SDK for ARM, where the
YieldProcessor function (in winnt.h) refers to _ARM_BARRIER_ISHST.

The actual MSVC armintr.h contains a lot more definitions, but this is enough to
build code that uses the Windows SDK but doesn't use ARM intrinsics directly.

An alternative would to just keep the addition to intrin.h (to include
armintr.h), but not actually ship armintr.h, instead having clang's intrin.h
include armintr.h from MSVC's include directory. (That one works fine with
clang, at least for building code that uses the Windows SDK.)

Patch by Martin Storsjö!

llvm-svn: 277928