This routine unlocks the Threads Library global mutex. Because
    the global mutex is recursive, the unlock occurs when each call
    to pthread_lock_global_np() has been matched by a call to this
    routine. For example, if you called pthread_lock_global_np()
    three times, pthread_unlock_global_np() unlocks the global mutex
    when you call it the third time.

    If no threads are waiting for the global mutex, it becomes
    unlocked with no current owner. If one or more threads are
    waiting to lock the global mutex, this routine causes one thread
    to unblock and to try to acquire the global mutex. The scheduling
    policy is used by this routine to determine which thread is
    awakened. For the policies SCHED_FIFO and SCHED_RR, a blocked
    thread is chosen in priority order, using first-in/first-out
    (FIFO) within priorities.