implement POSIX semaphores

src/thread/sem_post.c

@@ -0,0 +1,9 @@
+#include <semaphore.h>
+#include "pthread_impl.h"
+
+int sem_post(sem_t *sem)
+{
+	if (!a_fetch_add(sem->__val, 1))
+		__wake(sem->__val, 1, 0);
+	return 0;
+}