[llvm] [test] llvm-ar: Fix inadvertent unXFAIL for Apple platforms (PR #159796)

[Daniel Rodríguez Troitiño via llvm-commits]

https://github.com/drodriguez commented:

The problem is that the tests are liberal in using `--format=gnu` and it seems that the default format in macOS is probably `darwin` which seems to add newlines at the end of the files (I don't know why).

I think adding a couple of `--format=gnu` when creating the archives fixes the tests and should allow them to run on macOS no matter if you are targetting macOS or other platform, and no matter the triple spelling being used.

https://github.com/llvm/llvm-project/pull/159796