Allow use of spin_loop_hint

It's being deprecated/moved elsewhere. But we want to support old
compilers too.

Author: vorner

signal-hook-registry/src/half_lock.rs

@@ -181,6 +181,8 @@ impl<T> HalfLock<T> {
                 if iter % YIELD_EVERY == 0 {
                     thread::yield_now();
                 } else {
+                    // Replaced by hint::spin_loop, but we want to support older compiler
+                    #[allow(deprecated)]
                     atomic::spin_loop_hint();
                 }
             }

src/flag.rs

@@ -247,6 +247,8 @@ mod tests {
     fn wait_flag(flag: &AtomicBool) -> bool {
         let start = Instant::now();
         while !flag.load(Ordering::Relaxed) {
+            // Replaced by hint::spin_loop, but we want to support older compiler
+            #[allow(deprecated)]
             atomic::spin_loop_hint();
             if Instant::now() - start > Duration::from_secs(1) {
                 // We reached a timeout and nothing happened yet.