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

[Daniel Rodríguez Troitiño via llvm-commits]

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drodriguez wrote:

```
# RUN: llvm-ar rc --format=gnu %t/archive.a %t/a.txt %t/b.txt
```

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