(Ada) Add support for task switching when debugging core files

(Ada) Add support for task switching when debugging core files

The reasons for not supporting task switching when debugging core files
appear to now mostly be OBE. In particular, on GNU/Linux, the thread
layer is now able to retrieve the same thread info as in the live
process. So, this patch is mostly about just removing the guard
that limited the use of task switching to live processes.

gdb/ChangeLog:

        * ada-tasks.c (read_atcb): Properly set task_info->ptid
        when !target_has_execution as well.
        (task_command): Remove error when !target_has_execution.

gdb/testsuite/ChangeLog:

        * gdb.ada/task_switch_in_core: New testcase.