[libunwind] [libunwind] Fix exposure of UNW_AARCH64_RA_SIGN_STATE through _Unwind_GetGR (PR #209947)

[Nathan Corbyn via cfe-commits]

https://github.com/cofibrant commented:

I think I understand the objective and the approach makes sense. I'm not sure I follow what the vanilla test is doing, though.

https://github.com/llvm/llvm-project/pull/209947