x86: Tag some intrinsics with __pure2

Some C wrappers for x86 instructions do not touch global memory and only act
on their arguments; they can be marked pure2, aka const. Without this
annotation, Clang 3.9.1 is not intelligent enough on its own to grok that
these functions are const__.

Submitted by: Anton Rang <anton.rang AT isilon.com>
Sponsored by: Dell EMC Isilon