[lld] [llvm] [LLD][Docs] Document `-z gcs=` option in the man page (PR #146505)

[Oliver Stannard via llvm-commits]

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@@ -948,6 +948,15 @@ disallows overlap.
 .It Cm shstk
 x86 only, use shadow stack.
 .Pp
+.It Cm gcs Ns = Ns Ar [implicit|never|always]
+Specify how the GCS bit is set.
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ostannard wrote:

This could do with some introduction, most users won't know what "the GCS bit" is.

https://github.com/llvm/llvm-project/pull/146505