[PowerPC] Complete setting the baseline for ppc64le

Patch by Nemanja Ivanovic.

As was uncovered by the failing test case (when run on non-PPC
platforms), the feature set when compiling with -march=ppc64le was not
being picked up. This change ensures that if the -mcpu option is not
specified, the correct feature set is picked up regardless of whether
we are on PPC or not.

llvm-svn: 227455