Add -mqpx and -mno-qpx feature flags to toggle use of the PPC QPX vector instruction set

I've renamed the altivec test to ppc-features (because now there is more than one feature to test).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174204 91177308-0d34-0410-b5e6-96231b3b80d8