add missing feature gate for rustc_safe_intrinsic

Author: Noratrieb

compiler/rustc_feature/src/builtin_attrs.rs

@@ -740,6 +740,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         and it is only intended for internal use and as a desugaring."
     ),
 
+    rustc_attr!(
+        rustc_safe_intrinsic, AttributeType::Normal, template!(Word), ErrorFollowing,
+        "#[rustc_safe_intrinsics] declares intrinsics that are safe to use, tying it to the internal \
+        intrinsic mechanisms."
+    ),
+
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,
         // FIXME: This can be `true` once we always use `tcx.is_diagnostic_item`.