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

[Peter Smith 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]
+Specify how the GCS bit is set. A set GCS bit indicates that the object file supports the Guarded Control Stack security feature.
----------------
smithp35 wrote:

I think it would be better to say
```
Specify how the GNU_PROPERTY_AARCH64_FEATURE_1_GCS feature bit is set in the output ELF file. When set this declares that the ELF file supports the Guarded Control Stack (GCS) security feature.
```

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