[clang-cl] Add support for /arch:AVX512F and /arch:AVX512

For /arch:AVX512F:
clang-cl and cl.exe both defines __AVX512F__ __AVX512CD__.
clang-cl also defines __AVX512ER__ __AVX512PF__.
64-bit cl.exe also defines (according to /Bz) _NO_PREFETCHW.

For /arch:AVX512:
clang-cl and cl.exe both define
__AVX512F__ __AVX512CD__ __AVX512BW__ __AVX512DQ__ __AVX512VL__.
64-bit cl.exe also defines _NO_PREFETCHW.

So not 100% identical, but pretty close.

Also refactor the existing AVX / AVX2 code to not repeat itself in both the
32-bit and 64-bit cases.

https://reviews.llvm.org/D42538

llvm-svn: 323433