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

fusl/src/thread/pthread_rwlock_unlock.c

+#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;
+}