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

[Fangrui Song via llvm-commits]

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@@ -953,6 +960,17 @@ disallows overlap.
 .It Cm shstk
 x86 only, use shadow stack.
 .Pp
+.It Cm gcs Ns = Ns Ar [implicit|never|always]
+Specifies how the
+.Cm GNU_PROPERTY_AARCH64_FEATURE_1_GCS
+bit is set in the output ELF file. When set, it indicates support for the Guarded Control Stack (GCS) security feature.
+.Cm implicit
+(default) The GCS feature bit is set if all input relocatable files have a .note.gnu.property section containing the GNU_PROPERTY_AARCH64_FEATURE_1_GCS bit.
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MaskRay wrote:

.note.gnu.property also needs Cm

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