[PowerPC] Recognize long CPU name for -mtune in Clang

There are two ways of specifying a CPU on PowerPC:
power<N> and pwr<N>. Clang/LLVM traditionally
supports the latter and Clang replaces the former
with the latter when passing it to the back end for
the -mcpu= option. However, when the -mtune= option
was introduced, this replacement was not implemented for it.

This leaves us in an inconsistent state of accepting
both forms for -mcpu= and and only the latter for
-mtune=. Furthermore, it leaves us incompatible with
GCC which only understands the power<N> version for
both options.

This patch just adds the same handling for the long
names for -mtune= as already exists for -mcpu=.

Differential revision: https://reviews.llvm.org/D144967