[llvm] [AArch64] Restrict .variant_pcs directive to ELF targets (PR #138924)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Curious: Why is it that the Mach-O format does not get a `TargetStreamer`? Does it really need no target-specific support?

Presumably not, there's currently only ELF and Windows COFF `TargetStreamer`s. The ELF one does not add much (seems like mainly a few attributes) and the Windows one seems to only handle various unwinding instructions. I guess Mach-O handles these differently. 

https://github.com/llvm/llvm-project/pull/138924