[llvm] [llvm-ar] Fix darwin-related tests' XFAIL annotation (PR #130144)

[Lucas Duarte Prates via llvm-commits]

pratlucas wrote:

Thanks for the review! I did verify it on a Mac, both with a regular LLVM build and a cross-compiling downstream project (arm/arm-toolchain).

I've updated the PR title, which will be final commit message. Sorry about mixing it up, that's what I get for copy-pasting the commit message from #130138 :) 

https://github.com/llvm/llvm-project/pull/130144