libc: pthread: implement mutexes

lib/libc/pthread/mutex/pthread_mutex_lock.c

@@ -0,0 +1,24 @@
+#include <errno.h>
+#include <magenta/futex.h>
+#include <pthread.h>
+
+int pthread_mutex_lock(pthread_mutex_t *mut)
+{
+	kern_futex_t expected = 0;
+	while (1) {
+		expected = 0;
+		if (__atomic_compare_exchange_n(
+			    &mut->__v,
+			    &expected,
+			    1,
+			    0,
+			    __ATOMIC_ACQUIRE,
+			    __ATOMIC_ACQUIRE)) {
+			return 0;
+		}
+
+		futex_wait(&mut->__v, 1, FUTEX_PRIVATE);
+	}
+
+	return __set_errno(EAGAIN);
+}