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

fusl/src/thread/pthread_cleanup_push.c

Index: fusl/src/thread/pthread_cleanup_push.c
diff --git a/fusl/src/thread/pthread_cleanup_push.c b/fusl/src/thread/pthread_cleanup_push.c
new file mode 100644
index 0000000000000000000000000000000000000000..9b21764b56b0bea71300b30dedaf950248135112
--- /dev/null
+++ b/fusl/src/thread/pthread_cleanup_push.c
@@ -0,0 +1,20 @@
+#include "pthread_impl.h"
+
+static void dummy(struct __ptcb *cb)
+{
+}
+weak_alias(dummy, __do_cleanup_push);
+weak_alias(dummy, __do_cleanup_pop);
+
+void _pthread_cleanup_push(struct __ptcb *cb, void (*f)(void *), void *x)
+{
+	cb->__f = f;
+	cb->__x = x;
+	__do_cleanup_push(cb);
+}
+
+void _pthread_cleanup_pop(struct __ptcb *cb, int run)
+{
+	__do_cleanup_pop(cb);
+	if (run) cb->__f(cb->__x);
+}