[Ada] Do not compute task ptid when debugging core file

[Ada] Do not compute task ptid when debugging core file

After thread support over core files was added for GNU/Linux was added,
we started noticing the following type of crash when trying to perform
task switches (this is a bit accademic, since task switching is not
supported when debugging core files - this is what our testcase was
verifying).

(please check out the comment inside ada-tasks.c:task_command for
more details on this topic)

The reason for the crash comes from the fact that the GNU/Linux thread
layer now gets pushed on the target stack, causing the associated
to_get_ada_task_ptid target method to be activated.  This routine
makes the assumption that, for all threads, the private area is not
NULL.  This is incorrect in the case of core files, as the core layer
creates some threads with no private data.

But, taking a step back, we don't need to try to compute the task ptid,
as we'll never be using it anyways (we only use it for task switching).
So the fix is to avoid the ptid computation altogether when debugging
a core file.

gdb/ChangeLog:

        * ada-tasks.c (read_atcb): Do not compute the task ptid when
        debugging a core file.