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

fusl/src/thread/pthread_rwlock_timedrdlock.c

+#include "pthread_impl.h"
+
+int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rw, const struct timespec *restrict at)
+{
+        int r, t;
+
+        r = pthread_rwlock_tryrdlock(rw);
+        if (r != EBUSY) return r;
+        
+        int spins = 100;
+        while (spins-- && rw->_rw_lock && !rw->_rw_waiters) a_spin();
+
+        while ((r=pthread_rwlock_tryrdlock(rw))==EBUSY) {
+                if (!(r=rw->_rw_lock) || (r&0x7fffffff)!=0x7fffffff) continue;
+                t = r | 0x80000000;
+                a_inc(&rw->_rw_waiters);
+                a_cas(&rw->_rw_lock, r, t);
+                r = __timedwait(&rw->_rw_lock, t, CLOCK_REALTIME, at, rw->_rw_shared^128);
+                a_dec(&rw->_rw_waiters);
+                if (r && r != EINTR) return r;
+        }
+        return r;
+}