[libcxxabi] [libunwind] [libunwind][AIX] Handle VAPI-based return addresses in stack unwinding for LLU (PR #209280)

[Hubert Tong via cfe-commits]

https://github.com/hubert-reinterpretcast approved this pull request.

Functional changes LGTM but I think we want to figure out the testing.

https://github.com/llvm/llvm-project/pull/209280