Headers: be a bit more careful about inline asm

Conditionally include x86intrin.h if we are building for x86 or x86_64.
Conditionalise definition of inline assembly routines which use x86 or x86_64
inline assembly. This is needed as clang can target Windows on ARM where these
definitions may be included into user code.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@211716 91177308-0d34-0410-b5e6-96231b3b80d8