Bug#43435: LOCK_open does not use MY_MUTEX_INIT_FAST

Initialize LOCK_open as a adapative mutex on platforms where the
PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP macro is available. The flag
indicates that a thread should spin (busy wait) for some time on a
locked adaptive mutex before blocking (sleeping). It's intended to
to alleviate performance problems due to LOCK_open being a highly
contended mutex.

sql/mysqld.cc:
  Initialize LOCK_open as a adapative mutex.

sql/mysqld.cc

@@ -3519,7 +3519,7 @@ static int init_thread_environment()
   (void) pthread_mutex_init(&LOCK_mysql_create_db,MY_MUTEX_INIT_SLOW);
   (void) pthread_mutex_init(&LOCK_lock_db,MY_MUTEX_INIT_SLOW);
   (void) pthread_mutex_init(&LOCK_Acl,MY_MUTEX_INIT_SLOW);
-  (void) pthread_mutex_init(&LOCK_open, NULL);
+  (void) pthread_mutex_init(&LOCK_open, MY_MUTEX_INIT_FAST);
   (void) pthread_mutex_init(&LOCK_thread_count,MY_MUTEX_INIT_FAST);
   (void) pthread_mutex_init(&LOCK_mapped_file,MY_MUTEX_INIT_SLOW);
   (void) pthread_mutex_init(&LOCK_status,MY_MUTEX_INIT_FAST);