[NFC][PowerPC] Fix the check-cpu.ll test case.

This test doesn't work because the CHECK-NOT line is actually checking
something that only exists on stderr and not stdout.
Changed the test so that we now check both stderr and stdout.
Changed the test so that we check pwr9, pwr10, and future. The cpu names of
power9 or power10 are not supported in the llc backend.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D120349