[clang][PPC] Checking Unknown Values Passed to -mcpu

Currently `ppc::getPPCTargetCPU` returns an empty string when it encounters an unknown value passed to `-mcpu`. This causes `clang` to ignore unknown `-mcpu` values silently.

This patch changes the behaviour of `ppc::getPPCTargetCPU` so that it passes the unknown option to the target info, so the target info can actually check if the CPU string is supported, and report an error when encountering unknown/unsupported CPU string.

Reviewed By: jamieschmeiser

Differential Revision: https://reviews.llvm.org/D139720