MDEV-6489 - rpl.rpl_insert, rpl.rpl_insert_delayed and

            main.mysqlslap fail on PPC64

There seem to be a bug on Power8 which doesn't guarantee
a signal to be delivered to waiting thread if broadcast
is called outside of mutex.

For now workaround it by calling broadcast while mutex is
still held.

client/mysqlslap.c

@@ -1796,8 +1796,8 @@ run_scheduler(stats *sptr, statement *stmts, uint concur, ulonglong limit)
 
   pthread_mutex_lock(&sleeper_mutex);
   master_wakeup= 0;
-  pthread_mutex_unlock(&sleeper_mutex);
   pthread_cond_broadcast(&sleep_threshhold);
+  pthread_mutex_unlock(&sleeper_mutex);
 
   gettimeofday(&start_time, NULL);