[AIX] Avoid structor alias; die before bad alias codegen

Summary:
`AsmPrinter::emitGlobalIndirectSymbol` is dependent on
`MCStreamer::emitAssignment` to produce `.set` directives for alias
symbols; however, the `.set` pseudo-op on AIX is documented as not
usable with external relocatable terms or expressions, which limits its
applicability in generating alias symbols.

Disable generating aliases on AIX until a different implementation
strategy is available.

Reviewers: cebowleratibm, jasonliu, sfertile, daltenty, DiggerLin

Reviewed By: jasonliu

Differential Revision: https://reviews.llvm.org/D79044