Add more gcc compatibility names to clang's cpuid.h

Summary:
Some cpuid bit defines are named slightly different from how gcc's
cpuid.h calls them.

Define a few more compatibility names to appease software built for gcc:

* `bit_PCLMUL`      alias of `bit_PCLMULQDQ`
* `bit_SSE4_1`      alias of `bit_SSE41`
* `bit_SSE4_2`      alias of `bit_SSE42`
* `bit_AES`         alias of `bit_AESNI`
* `bit_CMPXCHG8B`   alias of `bit_CX8`

While here, add the misssing 29th bit, `bit_F16C` (which is how gcc
calls this bit).

Reviewers: joerg, rsmith

Subscribers: bruno, cfe-commits

Differential Revision: https://reviews.llvm.org/D22010

llvm-svn: 277307