[llvm] [PowerPC] Spill non-volatile registers required for traceback table (PR #71115)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 approved this pull request.

LGTM with some nits.

Thanks for your patience.

Would be great if you could wait for some days for other reviewers.

https://github.com/llvm/llvm-project/pull/71115