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

fusl/src/thread/pthread_rwlock_unlock.c

Index: fusl/src/thread/pthread_rwlock_unlock.c
diff --git a/fusl/src/thread/pthread_rwlock_unlock.c b/fusl/src/thread/pthread_rwlock_unlock.c
new file mode 100644
index 0000000000000000000000000000000000000000..7b5eec84d833c30673269e19b5467f2b13a05180
--- /dev/null
+++ b/fusl/src/thread/pthread_rwlock_unlock.c
@@ -0,0 +1,18 @@
+#include "pthread_impl.h"
+
+int pthread_rwlock_unlock(pthread_rwlock_t *rw)
+{
+	int val, cnt, waiters, new, priv = rw->_rw_shared^128;
+
+	do {
+		val = rw->_rw_lock;
+		cnt = val & 0x7fffffff;
+		waiters = rw->_rw_waiters;
+		new = (cnt == 0x7fffffff || cnt == 1) ? 0 : val-1;
+	} while (a_cas(&rw->_rw_lock, val, new) != val);
+
+	if (!new && (waiters || val<0))
+		__wake(&rw->_rw_lock, cnt, priv);
+
+	return 0;
+}