Correct hasFeature/isValidFeatureName's handling of shstk/adx/mwaitx

https://bugs.llvm.org/show_bug.cgi?id=35721 reports that x86intrin.h
is issuing a few warnings. This is because attribute target is using
isValidFeatureName for its source. It was also discovered that two of
these were missing from hasFeature.  

Additionally, shstk is and ibu are reordered alphabetically, as came
up during code review.

llvm-svn: 321324