Stabilize the compile_error_macro feature

Stabilizes:

* `compile_error!` as a macro defined by rustc

Closes #40872

src/libstd/macros.rs

@@ -249,7 +249,7 @@ pub mod builtin {
     /// For more information, see the [RFC].
     ///
     /// [RFC]: https://github.com/rust-lang/rfcs/blob/master/text/1695-add-error-macro.md
-    #[unstable(feature = "compile_error_macro", issue = "40872")]
+    #[stable(feature = "compile_error_macro", since = "1.20.0")]
     #[macro_export]
     macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) }