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

fusl/src/exit/at_quick_exit.c

+#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;
+}