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

fusl/src/thread/mtx_unlock.c

Index: fusl/src/thread/mtx_unlock.c
diff --git a/fusl/src/thread/mtx_unlock.c b/fusl/src/thread/mtx_unlock.c
new file mode 100644
index 0000000000000000000000000000000000000000..5033ace7f3292c9604e4f9627c3f912fba6aae87
--- /dev/null
+++ b/fusl/src/thread/mtx_unlock.c
@@ -0,0 +1,11 @@
+#include <threads.h>
+
+int __pthread_mutex_unlock(mtx_t *);
+
+int mtx_unlock(mtx_t *mtx)
+{
+	/* The only cases where pthread_mutex_unlock can return an
+	 * error are undefined behavior for C11 mtx_unlock, so we can
+	 * assume it does not return an error and simply tail call. */
+	return __pthread_mutex_unlock(mtx);
+}