[XCOFF][AIX] Add Global Variables Directly to TOC for 32 bit AIX

Summary:
This patch implements the backend implementation of adding global variables
directly to the table of contents (TOC), rather than adding the address of the
variable to the TOC.
Currently, this patch will look for the "toc-data" attribute on symbols in the
IR, and then add those symbols to the TOC.
ATM, this is implemented for 32 bit AIX.

Reviewers: sfertile
Differential Revision: https://reviews.llvm.org/D101178