[llvm] [AArch64] Null check TargetStreamer before emitting .variant_pcs (PR #138924)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

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

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