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

[Fangrui Song via llvm-commits]

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@@ -798,6 +798,11 @@ Specify how to report the missing GNU_PROPERTY_X86_FEATURE_1_IBT or GNU_PROPERTY
 .Cm none
 is the default, linker will not report the missing property otherwise will be reported as a warning or an error.
 .Pp
+.It Cm gcs-report Ns = Ns Ar [none|warning|error]
+Specify how to report missing GNU_PROPERTY_AARCH64_FEATURE_1_GCS property. GNU_PROPERTY_AARCH64_FEATURE_1_GCS indicates object file support for the Guarded Control Stack security feature.
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MaskRay wrote:

`GNU_PROPERTY_AARCH64_FEATURE_1_*` properties need to use `.Cm`

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