[llvm] [AArch64] Restrict .variant_pcs directive to ELF targets (PR #140022)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm approved this pull request.

This indeed seems a duplicate of #138924. Since this version addresses already the [comment](https://github.com/llvm/llvm-project/pull/138924/files#r2084031408) I left on the other PR, I'm happy to accept it if you address my nit on the test. Thanks!

https://github.com/llvm/llvm-project/pull/140022