[XCOFF][AIX] Handle llvm.used and llvm.compiler.used global array

For now, just return and do nothing when we see llvm.used and
llvm.compiler.used global array.
Hopefully, we could come up with a good solution later to prevent
linker from eliminating symbols in llvm.used array.

Reviewed By: DiggerLin, daltenty

Differential Revision: https://reviews.llvm.org/D84363