cheap special-case optimization for normal mutexes

cycle-level benchmark on atom cpu showed typical pthread_mutex_lock
call dropping from ~120 cycles to ~90 cycles with this change. benefit
may vary with compiler options and version, but this optimization is
very cheap to make and should always help some.

src/thread/pthread_mutex_lock.c

@@ -3,6 +3,10 @@
 int pthread_mutex_lock(pthread_mutex_t *m)
 {
 	int r;
+
+	if (m->_m_type == PTHREAD_MUTEX_NORMAL && !a_swap(&m->_m_lock, 1))
+		return 0;
+
 	while ((r=pthread_mutex_trylock(m)) == EBUSY) {
 		if (!(r=m->_m_lock) || (r&0x40000000)) continue;
 		if ((m->_m_type&3) == PTHREAD_MUTEX_ERRORCHECK