[lld] [LLD][Docs] Document -z gcs= option in the man page (PR #146522)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM thanks for the updates. Please leave a few days before merging to give @MaskRay a chance to comment.

https://github.com/llvm/llvm-project/pull/146522