Update the description of link_cfg unstable

Fmt code

Update tests

Modify msg

Signed-off-by: Rustin-Liu <rustin.liu@gmail.com>

src/librustc_metadata/native_libs.rs

@@ -162,7 +162,12 @@ impl Collector<'tcx> {
             }
         }
         if lib.cfg.is_some() && !self.tcx.features().link_cfg {
-            feature_err(&self.tcx.sess.parse_sess, sym::link_cfg, span.unwrap(), "is unstable")
+            feature_err(
+                &self.tcx.sess.parse_sess,
+                sym::link_cfg,
+                span.unwrap(),
+                "kind=\"link_cfg\" is unstable",
+            )
             .emit();
         }
         if lib.kind == cstore::NativeStaticNobundle && !self.tcx.features().static_nobundle {

src/test/ui/feature-gates/feature-gate-link_cfg.stderr

@@ -1,4 +1,4 @@
-error[E0658]: is unstable
+error[E0658]: kind="link_cfg" is unstable
   --> $DIR/feature-gate-link_cfg.rs:1:1
    |
 LL | #[link(name = "foo", cfg(foo))]