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

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> Is this similar to #138924? @MacDue

seems so! Apologies I wasn't aware there was a fix in progress already.. I came across this via #138260 the other day and there's no link to the existing PR, perhaps @MacDue wasn't aware there's an existing issue.

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