Stabilise repr128

compiler/rustc_error_codes/src/error_codes/E0658.md

@@ -3,10 +3,7 @@ An unstable feature was used.
 Erroneous code example:
 
 ```compile_fail,E0658
-#[repr(u128)] // error: use of unstable library feature 'repr128'
-enum Foo {
-    Bar(u64),
-}
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
 ```
 
 If you're using a stable or a beta version of rustc, you won't be able to use
@@ -17,12 +14,9 @@ If you're using a nightly version of rustc, just add the corresponding feature
 to be able to use it:
 
 ```
-#![feature(repr128)]
+#![feature(core_intrinsics)]
 
-#[repr(u128)] // ok!
-enum Foo {
-    Bar(u64),
-}
+use std::intrinsics; // ok!
 ```
 
 [rustup]: https://rust-lang.github.io/rustup/concepts/channels.html