Relax memory ordering used in SameMutexCheck

`SameMutexCheck` only requires atomicity for `self.addr`, but does not need ordering of other memory accesses in either the success or failure case. Using `Relaxed`, the code still correctly handles the case when two threads race to store an address.

Author: Austin Kiekintveld

library/std/src/sys_common/condvar/check.rs

@@ -24,7 +24,7 @@ impl SameMutexCheck {
     }
     pub fn verify(&self, mutex: &MovableMutex) {
         let addr = mutex.raw() as *const imp::Mutex as *const () as *mut _;
-        match self.addr.compare_exchange(ptr::null_mut(), addr, Ordering::SeqCst, Ordering::SeqCst)
+        match self.addr.compare_exchange(ptr::null_mut(), addr, Ordering::Relaxed, Ordering::Relaxed)
         {
             Ok(_) => {}               // Stored the address
             Err(n) if n == addr => {} // Lost a race to store the same address