util: add avx2 and xop detection to cpu detection code

util: add avx2 and xop detection to cpu detection code

Going to need this soon (not going to bother with avx2 intrinsics at this time
but don't want to do workarounds for true vector shifts if llvm itself can use
them just fine and won't need the gazillion instruction emulation).
Not really tested other than my cpu returns 0 for these features...
(I have no idea if llvm actually would emit avx2/xop instructions neither...)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>