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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> > 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!
> 
> Appreciate it, but Ben got there first so he can have this one.

I think this version of this fix is more correct than mine, so I'll take version this and add you as a co-author on my PR :+1: 

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