Add #[repr(i8)] to Ordering

Followup to rust-lang#89491 to allow `Ordering` to auto-derive `AsRepr` once
the proposal to add `AsRepr` (rust-lang#81642) lands.

Author: lopopolo

core/src/cmp.rs

@@ -323,6 +323,7 @@ pub struct AssertParamIsEq<T: Eq + ?Sized> {
 /// ```
 #[derive(Clone, Copy, PartialEq, Debug, Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(i8)]
 pub enum Ordering {
     /// An ordering where a compared value is less than another.
     #[stable(feature = "rust1", since = "1.0.0")]