[NFC][PPC][AIX] Add stack frame layout diagram to PPCISelLowering.cpp

Summary:
This NFC patch adds a diagram of the AIX ABI stack frame layout.

Based on https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/assembler/idalangref_runtime_process.html

Reviewers: sfertile, cebowleratibm, hubert.reinterpretcast, Xiangling_L

Reviewed By: sfertile

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

Tags: #powerpc, #llvm

Differential Revision: https://reviews.llvm.org/D82408