[libunwind] [libunwind] Add GCS support for AArch64 (PR #99335)

[Nico Weber via cfe-commits]

nico wrote:

That sounds believable.

We did update our compiler just an hour or so ago, so it's possible it will work for us now.

However, it suggests that the assumption `defined(__ARM_FEATURE_GCS_DEFAULT)` => arm_acle.h has `__chkfeat()` isn't valid in this snippet in libunwind/src/cet_unwind.h (https://github.com/llvm/llvm-project/pull/99335/files#diff-7c503795ceadc957122b87cbf0dff76624c0576c291268226952dd822c75ea6eR41)

```
#if defined(_LIBUNWIND_TARGET_AARCH64) && defined(__ARM_FEATURE_GCS_DEFAULT)
#define _LIBUNWIND_USE_GCS 1
#include <arm_acle.h>
```

https://github.com/llvm/llvm-project/pull/99335