[AIX] Emit TOC entries for ASM printing

Summary:
Emit the correct .toc psuedo op when we change to the TOC and emit
TC entries. Make sure TOC psuedos get the right symbols via overriding
getMCSymbolForTOCPseudoMO on AIX. Add a test for TOC assembly writing
and update tests to include TOC entries.

Also make sure external globals have a csect set and handle external function descriptor (originally authored by Jason Liu) so we can emit TOC entries for them.

Reviewers: DiggerLin, sfertile, Xiangling_L, jasonliu, hubert.reinterpretcast

Reviewed By: jasonliu

Subscribers: arphaman, wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70461