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

fusl/src/exit/at_quick_exit.c

Index: fusl/src/exit/at_quick_exit.c
diff --git a/fusl/src/exit/at_quick_exit.c b/fusl/src/exit/at_quick_exit.c
new file mode 100644
index 0000000000000000000000000000000000000000..34541badad445160b743c6fe77bee0117f04bf49
--- /dev/null
+++ b/fusl/src/exit/at_quick_exit.c
@@ -0,0 +1,29 @@
+#include <stdlib.h>
+#include "libc.h"
+
+#define COUNT 32
+
+static void (*funcs[COUNT])(void);
+static int count;
+static volatile int lock[2];
+
+void __funcs_on_quick_exit()
+{
+	void (*func)(void);
+	LOCK(lock);
+	while (count > 0) {
+		func = funcs[--count];
+		UNLOCK(lock);
+		func();
+		LOCK(lock);
+	}
+}
+
+int at_quick_exit(void (*func)(void))
+{
+	if (count == 32) return -1;
+	LOCK(lock);
+	funcs[count++] = func;
+	UNLOCK(lock);
+	return 0;
+}