[libunwind] [libunwind] fix unwinding from signal handler (PR #92291)

[Azat Khuzhin via cfe-commits]

azat wrote:

>[Build and Test libc++ / stage3 (generic-ubsan, libcxx-runners-8-set) (pull_request) Failing after 10m](https://github.com/llvm/llvm-project/actions/runs/9099207645/job/25013031369?pr=92291)

Looks like unrelated?

```
2024-05-15T17:48:22.3468783Z ##[error]The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
```

https://github.com/llvm/llvm-project/pull/92291