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

[Fangrui Song via llvm-commits]

================
@@ -953,6 +958,15 @@ disallows overlap.
 .It Cm shstk
 x86 only, use shadow stack.
 .Pp
+.It Cm gcs Ns = Ns Ar [implicit|never|always]
+Specifies how the 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) GCS feature bit is inferred from the input object marking.
----------------
MaskRay wrote:

`input object marking` is vague. I'd emphasize relocatable files. Perhaps something  like

When all relocatable files contain a .note.gnu.property section containing the GNU_PROPERTY_AARCH64_FEATURE_1_GCS bit


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