[libunwind] [libunwind] Fix aarch64 unwinding with a debugger attached (PR #162867)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

CC @cdavis5e @ssaba @corsix

I tried to summarize the issue and the fix to the best of my ability in the PR description/commit message - does it seem correct?

https://github.com/llvm/llvm-project/pull/162867