Testing for the fix for bug 23429.

Follow-up to commit for revision 236848.
Just a test case for the macro definition under the right CPU/Arch.
One combination was actually missed in the initial fix:
  - powerpc64-unknown-unknown -mcpu=pwr8 (rather than -mcpu=power8).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237386 91177308-0d34-0410-b5e6-96231b3b80d8