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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

I've pushed your patch to https://github.com/llvm/llvm-project/pull/138924 (and addressed @sdesmalen-arm's nit).

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