[llvm] [Dexter] Extend XFAIL of Dexter tests to all MacOS architectures. (PR #83936)

[via llvm-commits]

dyung wrote:

> Can we just skip the test instead?

Do you mean by adding something like `// REQUIRES: !system-darwin`?

I would prefer to fix my bots so that they can correctly run the tests, but skipping the test is preferred to having them pass/fail on different configurations.

https://github.com/llvm/llvm-project/pull/83936