Apply suggestions from review pt.2

Co-authored-by: Vadim Petrochenkov <[email protected]>

Author: zetanumbers

tests/ui/feature-gates/feature-gate-link-arg-attribute.rs

@@ -1,5 +1,5 @@
 #[link(kind = "link-arg", name = "foo")]
-//~^ ERROR: is unstable
+//~^ ERROR link kind `link-arg` is unstable
 extern "C" {}
 
 fn main() {}