This is cute, but why not just do this with gdb? All-stop mode is on by default so it will stop all threads when you attach, and then you can simply invoke the exit_group() syscall.

    tty1$ ./program
    
    tty2$ gdb attach $(ps aux | grep program | head -1 | awk '{ print $2 }')
    Attaching to process 708267
    [New LWP 708268]
    [New LWP 708269]
    [New LWP 708270]
    [New LWP 708271]
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
    0x00007f00b1633117 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
    
    (gdb) p (long)syscall(231, 0)
    [Thread 0x7f00565e6640 (LWP 708271) exited]
    [Thread 0x7f005a5e8640 (LWP 708270) exited]
    [Thread 0x7f005ade9640 (LWP 708269) exited]
    [Thread 0x7f005d5ee640 (LWP 708268) exited]
    [Inferior 1 (process 708267) exited normally]
    The program being debugged exited while in a function called from GDB.
    Evaluation of the expression containing the function
    (syscall) will be abandoned.
    (gdb) quit
    
    tty1$ echo $?
    0