Make Barrier RefUnwindSafe again

This commit manually implements `RefUnwindSafe` for
`std::sync::Barrier` to fix 146087. This is a fix for a regression
indroduced by e95db591a4

library/std/src/sync/barrier.rs

@@ -1,3 +1,5 @@
+use core::panic::RefUnwindSafe;
+
 use crate::fmt;
 use crate::sync::nonpoison::{Condvar, Mutex};
 
@@ -31,6 +33,9 @@ pub struct Barrier {
     num_threads: usize,
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl RefUnwindSafe for Barrier {}
+
 // The inner state of a double barrier
 struct BarrierState {
     count: usize,

library/std/tests/sync/barrier.rs

@@ -1,3 +1,4 @@
+use std::panic::RefUnwindSafe;
 use std::sync::mpsc::{TryRecvError, channel};
 use std::sync::{Arc, Barrier};
 use std::thread;
@@ -33,3 +34,12 @@ fn test_barrier() {
     }
     assert!(leader_found);
 }
+
+#[expect(dead_code, reason = "this is essentially a compile pass test")]
+fn check_barrier_is_ref_unwind_safe() {
+    let barrier = Arc::new(Barrier::new(10));
+
+    fn check<T: RefUnwindSafe>(_: T) {}
+
+    check(barrier);
+}