Add cpu feature detection support for PPC

In #21849 Yichao added infrastructure for cpu feature detection for ARM and X86, there is a fallback implementation that  is enough to get PPC running,
but with Power9 around the corner we should add cpu detection for PPC as well.

Currently also causes a test-failure in `cmdlineargs` since `julia --cpu-target=invalidtarget` doesn't fail.

cc: #26218 