[PowerPC][NFC] Power ISA features for Semachecking

[NFC] This patch adds features for pwr7, pwr8, and pwr9 that can be
used for semachecking builtin functions that are only valid for certain
versions of ppc.

Reviewed By: nemanjai, #powerpc
Authored By: Quinn Pham <Quinn.Pham@ibm.com>

Differential revision: https://reviews.llvm.org/D105501