[ADT] Recognize ppc as valid architecture in target triple.

Until this patch, only `powerpc` and `ppc32` were recognized as valid
PowerPC 32-bit architectures in a target triple. This was incompatible
with the triple `ppc-apple-darwin` as returned for libObject. I found
out about this when working on a test case using a binary generated on
an old PowerBook G4.

We had the choice of either fix this in the Mach-O object parser or
in the Triple implementation. I chose the latter because it feels like
the most canonical place.

Differential revision: https://reviews.llvm.org/D43760

llvm-svn: 326182