[gdb/testsuite] Fix gdb.ada/mi_task_arg.exp for -m32

[gdb/testsuite] Fix gdb.ada/mi_task_arg.exp for -m32

When running test-case gdb.ada/mi_task_arg.exp with target board unix/-m32, I
run into:
...
(gdb) ^M
Expecting: ^(-stack-list-arguments 1[^M
]+)?(\^done,stack-args=\[ \
  frame={level="0",args=\[\]}, \
  frame={level="1",args=\[{name="<_task>",value="0x[0-9A-Fa-f]+"}\]}, \
  frame={level="2",args=\[({name="self_id",value="0x[0-9A-Fa-f]+"})?\]},.*[^M
]+[(]gdb[)] ^M
[ ]*)
-stack-list-arguments 1^M
^done,stack-args=[ \
  frame={level="0",args=[]}, \
  frame={level="1",args=[{name="<_task>",value="0x808abf0"}]}, \
  frame={level="2",args=[{name="self_id",value="<optimized out>"}]}, \
  frame={level="3",args=[]},frame={level="4",args=[]}]^M
(gdb) ^M
FAIL: gdb.ada/mi_task_arg.exp: -stack-list-arguments 1 (unexpected output)
...

The problem is that we're expecting a $hex for the value of self_id, but
instead get <optimized out>.

Looking at the debug info for self_id:
...
 <1><12a1f>: Abbrev Number: 84 (DW_TAG_subprogram)
    <12a20>   DW_AT_name        : system__tasking__stages__task_wrapper
    ...
 <2><12a35>: Abbrev Number: 61 (DW_TAG_formal_parameter)
    <12a36>   DW_AT_name        : self_id
    <12a40>   DW_AT_location    : 0x459e (location list)
...
it refers to location information here:
...
    0000459e 08053060 080531ac (DW_OP_fbreg: 0)
    000045aa 0805327c 080532a5 (DW_OP_fbreg: 0)
    000045b6 08053320 08053324 (DW_OP_fbreg: 0)
...
while the pc used to retrieve the location information is 0x080531c5:
...
 $ gdb -batch outputs/gdb.ada/mi_task_arg/task_switch \
   -ex "break 57" -ex run -ex bt
   ...
 #0  task_switch.break_me () at task_switch.adb:57
 #1  0x0804aaae in task_switch.caller (<_task>=0x808abf0) \
                     at task_switch.adb:51
 #2  0x080531c5 in system.tasking.stages.task_wrapper \
                     (self_id=<optimized out>) at s-tassta.adb:1295
...
which indeed falls outside of the ranges listed in the location info.

Fix this by accepting <optimized out> as valid value of self_id.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-08  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/mi_task_arg.exp: Accept <optimized out> as valid value of
	self_id.