UserHandler contains ancient LinuxThreads code

    // 4511530 - sem_post is serialized and handled by the manager thread. When
    // the program is interrupted by Ctrl-C, SIGINT is sent to every thread. We
    // don't want to flood the manager thread with sem_post requests.
    if (sig == SIGINT && Atomic::add(1, &sigint_count) > 1)
      return;

This is referring to how the ancient LinuxThreads implementation worked. It is irrelevant with NPTL threading and should never have been copied into the code for the other OS.