compiler: adjust error formatting for extern "ABI" error

Author: workingjubilee

Diff for: compiler/rustc_ast_lowering/src/stability.rs

@@ -58,10 +58,13 @@ impl fmt::Display for UnstableAbi {
         let name = name.trim_matches('"');
         match self.explain {
             GateReason::Experimental => {
-                write!(f, "{name} ABI is experimental and subject to change")
+                write!(f, r#"the extern "{name}" ABI is experimental and subject to change"#)
             }
             GateReason::ImplDetail => {
-                write!(f, "{name} ABI is an implementation detail and perma-unstable")
+                write!(
+                    f,
+                    r#"the extern "{name}" ABI is an implementation detail and perma-unstable"#
+                )
             }
         }
     }