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

fusl/src/thread/pthread_rwlock_tryrdlock.c

Index: fusl/src/thread/pthread_rwlock_tryrdlock.c
diff --git a/fusl/src/thread/pthread_rwlock_tryrdlock.c b/fusl/src/thread/pthread_rwlock_tryrdlock.c
new file mode 100644
index 0000000000000000000000000000000000000000..fa271fcc6a156a78e9dfe975bf084113e3aa3167
--- /dev/null
+++ b/fusl/src/thread/pthread_rwlock_tryrdlock.c
@@ -0,0 +1,13 @@
+#include "pthread_impl.h"
+
+int pthread_rwlock_tryrdlock(pthread_rwlock_t *rw)
+{
+	int val, cnt;
+	do {
+		val = rw->_rw_lock;
+		cnt = val & 0x7fffffff;
+		if (cnt == 0x7fffffff) return EBUSY;
+		if (cnt == 0x7ffffffe) return EAGAIN;
+	} while (a_cas(&rw->_rw_lock, val, val+1) != val);
+	return 0;
+}