Un-unsafe the StableOrd trait

Whilst incorrect implementations of this trait can cause miscompilation,
they cannot cause memory unsafety in rustc.

compiler/rustc_abi/src/lib.rs

@@ -425,10 +425,10 @@ pub struct Size {
     raw: u64,
 }
 
-// Safety: Ord is implement as just comparing numerical values and numerical values
+// Ord is implement as just comparing numerical values and numerical values
 // are not changed by (de-)serialization.
 #[cfg(feature = "nightly")]
-unsafe impl StableOrd for Size {
+impl StableOrd for Size {
     const CAN_USE_UNSTABLE_SORT: bool = true;
 }