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

[Peter Smith via llvm-commits]

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@@ -953,6 +958,15 @@ 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. A set GCS bit indicates that the object file supports the Guarded Control Stack security feature.
+.Cm implicit
+is the default, where the GCS bit is inferred from the input objects.
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smithp35 wrote:

>From looking at noseparate-code above. I suggest
```(default) feature bit is inferred from the input object marking.```

I've suggested feature bit rather than GCS bit as we've used the full `GNU_PROPERTY_AARCH64_FEATURE_1_GCS feature bit` above.

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