Modernize the aix-thread later by getting rid of the base_target

    Modernize the aix-thread later by getting rid of the base_target
        global.  This brings back to life the AIX port which was otherwise
        crashing all the time.

        * aix-thread.c (base_target): Delete.
        (pd_enable): Do not set base_target.
        (aix_thread_attach): Use find_target_beneath instead of base_target.
        (aix_thread_detach, aix_thread_resume, aix_thread_wait)
        (aix_thread_fetch_registers, aix_thread_store_registers),
        (aix_thread_xfer_partial, aix_thread_mourn_inferior)
        (aix_thread_thread_alive, aix_thread_pid_to_str): Likewise.
        (aix_thread_kill): Delete. Does not seem necessary.
        (init_aix_thread_ops): Do not set aix_thread_ops.to_kill.