[AIX][PowerPC] Define __powerpc and __PPC macros

%%%
This patch defines the macros __powerpc and __PPC on AIX to be consistent with XL for AIX. See: https://www.ibm.com/docs/en/xl-c-and-cpp-aix/13.1.0?topic=macros-related-platform

Note: GCC does not currently define __powerpc and __PPC so users should prefer the __powerpc__ and __PPC__ forms.
%%%

Reviewed By: cebowleratibm

Differential Revision: https://reviews.llvm.org/D108917