[PPC][AIX] Implement variadic function handling in LowerFormalArguments_AIX

Summary:
This patch adds support for handling of variadic functions for AIX.
This includes ensuring that use and consume correct type of
va_list (char *va_list) for AIX.

Authored by: ZarkoCA

Reviewers: cebowleratibm, sfertile, jasonliu

Reviewed by: jasonliu

Differential Revision: https://reviews.llvm.org/D76130