core: unstably expose atomic_compare_exchange so stdarch can use it

Author: RalfJung

library/core/src/sync/atomic.rs

@@ -3885,10 +3885,13 @@ unsafe fn atomic_sub<T: Copy>(dst: *mut T, val: T, order: Ordering) -> T {
     }
 }
 
+/// Publicly exposed for stdarch; nobody else should use this.
 #[inline]
 #[cfg(target_has_atomic)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-unsafe fn atomic_compare_exchange<T: Copy>(
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[doc(hidden)]
+pub unsafe fn atomic_compare_exchange<T: Copy>(
     dst: *mut T,
     old: T,
     new: T,