[lld] [LLD] [ELF] Make -z gcs=always implicitly warn on missing GCS, like force-bti (PR #186203)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Thanks for the review! Do you think this is reasonable for the 22.x branch?

I think this is acceptable, but hope @smithp35 can confirm.

https://github.com/llvm/llvm-project/pull/186203