[AIX] Return the correct set of callee saved regs

Summary:
r13 isn't reserved on 32-bit AIX, which is reflected in our calling
convention but not callee saved regs.

Reviewers: sfertile, ZarkoCA, cebowleratibm, jasonliu

Reviewed By: sfertile

Subscribers: thakis, lei, wuzish, nemanjai, hiraditya, kbarton, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77101