Use allow(unused_imports) instead of cfg(doc)

This prevents links from breaking when items are re-exported in a
different crate and the original isn't being documented.

Author: jyn514

library/core/src/intrinsics.rs

@@ -58,7 +58,7 @@ use crate::marker::DiscriminantKind;
 use crate::mem;
 
 // These imports are used for simplifying intra-doc links
-#[cfg(doc)]
+#[allow(unused_imports)]
 use crate::sync::atomic::{self, AtomicBool, AtomicI32, AtomicIsize, AtomicU32, Ordering};
 
 #[stable(feature = "drop_in_place", since = "1.8.0")]