Add pluralise macro

Adress issue #64238. Create a macro to be used for pluralisation check
throughout rustc codebase.

src/librustc_errors/lib.rs

@@ -845,3 +845,11 @@ impl Level {
         }
     }
 }
+
+#[macro_export]
+macro_rules! pluralise {
+            ($x:expr) => {
+                if $x != 1 { "s" } else { "" }
+            };
+}
+