Fix core AsyncFn stability attribute

Author: ehuss

library/core/src/prelude/common.rs

@@ -12,7 +12,8 @@ pub use crate::marker::{Copy, Send, Sized, Sync, Unpin};
 #[stable(feature = "core_prelude", since = "1.4.0")]
 #[doc(no_inline)]
 pub use crate::ops::{Drop, Fn, FnMut, FnOnce};
-#[stable(feature = "async_closure", since = "1.85.0")]
+#[cfg_attr(bootstrap, unstable(feature = "async_closure", issue = "62290"))]
+#[cfg_attr(not(bootstrap), stable(feature = "async_closure", since = "1.85.0"))]
 #[doc(no_inline)]
 pub use crate::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};