[TargetInfo] Sort target features before passing them to the backend

Passing the features in random order will lead to unpredictable results
when some of the features are related (like the architecture-version
features on ARM).

It might be possible to fix this particular case in the ARM target code,
to avoid adding overlapping target features. But we should probably be
sorting in any case: the behavior shouldn't depend on StringMap's
hashing algorithm.

Differential Revision: https://reviews.llvm.org/D46030

llvm-svn: 330861