[libunwind] [libunwind] XFAIL the za unwind test on Apple targets older than OS 27.0 (PR #211379)

[Louis Dionne via cfe-commits]

ldionne wrote:

This should resolve the intermittent failures we're seeing in the CI when the self-hosted runner host that's selected happens to support SME, for the `apple-system` and `apple-system-hardened` jobs (both of which are back-deployment tests running against the system libunwind).

https://github.com/llvm/llvm-project/pull/211379