[PATCH] Implement __readeflags and __writeeflags intrinsics

[Alexey Volkov]

  These intrinsics in GCC are in ia32intrin.h header.
  There are no such file in clang headers.
  Looking through functions in GCC's ia32intrin.h I found similar ones in Clang's Intrin.h file and other headers included into x86intrin.h.
  So we could use some file included into x86intrin.h or create a new header and include it into x86intrin.h.

http://llvm-reviews.chandlerc.com/D2468