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

fusl/src/thread/mtx_lock.c

Index: fusl/src/thread/mtx_lock.c
diff --git a/fusl/src/thread/mtx_lock.c b/fusl/src/thread/mtx_lock.c
new file mode 100644
index 0000000000000000000000000000000000000000..5c2415c1acdbc5ee1348da1cf43c71557591d8ab
--- /dev/null
+++ b/fusl/src/thread/mtx_lock.c
@@ -0,0 +1,12 @@
+#include "pthread_impl.h"
+#include <threads.h>
+
+int mtx_lock(mtx_t *m)
+{
+	if (m->_m_type == PTHREAD_MUTEX_NORMAL && !a_cas(&m->_m_lock, 0, EBUSY))
+		return thrd_success;
+	/* Calling mtx_timedlock with a null pointer is an extension.
+	 * It is convenient, here to avoid duplication of the logic
+	 * for return values. */
+	return mtx_timedlock(m, 0);
+}