[PowerPC] Fix powerpcspe subtarget enablement in llvm backend

Summary:
As currently written, -target powerpcspe will enable SPE regardless of
disabling the feature later on in the command line.  Instead, change
this to just set a default CPU to 'e500' instead of a generic CPU.

As part of this, add FeatureSPE to the e500 definition.

Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D72673