CRYPTO_atomic_add(): use acquire release memory order rather than relaxed

For increments, the relaxed model is fine.  For decrements, it's
recommended to use the acquire release model.  We therefore go for the
latter.
Reviewed-by: NAndy Polyakov <appro@openssl.org>

crypto/threads_pthread.c

@@ -109,8 +109,8 @@ int CRYPTO_THREAD_compare_id(CRYPTO_THREAD_ID a, CRYPTO_THREAD_ID b)
 
 int CRYPTO_atomic_add(int *val, int amount, int *ret, CRYPTO_RWLOCK *lock)
 {
-# if defined(__GNUC__) && defined(__ATOMIC_RELAXED)
-    *ret = __atomic_add_fetch(val, amount, __ATOMIC_RELAXED);
+# if defined(__GNUC__) && defined(__ATOMIC_ACQ_REL)
+    *ret = __atomic_add_fetch(val, amount, __ATOMIC_ACQ_REL);
 # else
     if (!CRYPTO_THREAD_write_lock(lock))
         return 0;