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=.