[RFA/Ada] Implement Ada tasking support (take 2)

[Joel Brobecker]

>         * target.h (struct target_ops): Add new field to_get_ada_task_ptid.
>         (target_get_ada_task_ptid): New macro.
>         * target.c (default_get_ada_task_ptid): New function.
>         (update_current_target): Inherit field default_get_ada_task_ptid.
>         (update_current_target): Make default_get_ada_task_ptid the default
>         value for field to_get_ada_task_ptid.

Just wondering if anyone had any objection to this part.  Once I have
this part ok'ed, the rest is Ada-specific, does not affect the rest
of the code, and is pretty clean (IMHO - which doesn't mean it can't
be improved). I would prefer if this part didn't linger too much;
otherwise, I have to resync the patch, resend, etc.

Thank you,
-- 
Joel