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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

Previous version of this PR, before re-stacking as a stacked-PR: https://github.com/llvm/llvm-project/pull/205188

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