Implement thread support with core files on alpha-tru64

Thread support currently does not work with core files.  Note that,
in order to thread support to work on tru64, one need to allow GDB
to write in the core file (this is because the thread debug library
needs to write).

An obvious visible symptom of the problem is that "info threads" does
not list the various threads:

    (gdb) info threads
    * 1 <main task>  0x000003ff805c0918 in __nxm_thread_kill ()
       from /usr/shlib/libpthread.so

One other noticeable consequence is that GDB generates some warnings
when using "info tasks":

    (gdb) info tasks
    warning: Could not find thread id from THREAD = 0x3ffc01b6000

    warning: Could not find thread id from THREAD = 0x20000e2b4c0

      ID       TID P-ID Pri State                  Name
    *  1 140051000    0  30 Runnable               main_task
    *  2 14005c000    1  30 Accept or Select Term  my_t

(notice also how both tasks are marked as being the active task,
which cannot be true).

The problem is that the dec-thread module has not updated its thread list
after the core file got loaded.  In fact, the list only gets resync'ed
at the end of each target-wait.  The solution was to implement the
find_new_threads target_ops method.

gdb/ChangeLog:

        Implement thread support with core files on alpha-tru64.
        * dec-thread.c (dec_thread_find_new_threads): New function,
        extracted from resync_thread_list.
        (resync_thread_list): Add OPS parameter.  Replace extracted-out
        code by call to dec_thread_find_new_threads.
        (dec_thread_wait): Update call to resync_thread_list.
        (init_dec_thread_ops): Set dec_thread_ops.to_find_new_threads.

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