Add a "fork" of musl as //fusl.

fusl/src/thread/sem_trywait.c

Index: fusl/src/thread/sem_trywait.c
diff --git a/fusl/src/thread/sem_trywait.c b/fusl/src/thread/sem_trywait.c
new file mode 100644
index 0000000000000000000000000000000000000000..04edf46b524de72a575b1db7687f56d93fc1e291
--- /dev/null
+++ b/fusl/src/thread/sem_trywait.c
@@ -0,0 +1,13 @@
+#include <semaphore.h>
+#include "pthread_impl.h"
+
+int sem_trywait(sem_t *sem)
+{
+	int val;
+	while ((val=sem->__val[0]) > 0) {
+		int new = val-1-(val==1 && sem->__val[1]);
+		if (a_cas(sem->__val, val, new)==val) return 0;
+	}
+	errno = EAGAIN;
+	return -1;
+}