[gdb/testsuite] Fix gdb.ada/mi_task_arg.exp

[gdb/testsuite] Fix gdb.ada/mi_task_arg.exp

On openSUSE Leap 15.1, we have:
...
FAIL: gdb.ada/mi_task_arg.exp: -stack-list-arguments 1 (unexpected output)
...

The problem is that the stack-list-arguments command prints a frame argument
'self_id' for function system.tasking.stages.task_wrapper:
...
frame={level="2",args=[{name="self_id",value="0x12345678"}]
...
where none (args=[]) is expected.

The frame argument is in fact correct.  The FAIL does not show for say, fedora
30, because there the executable uses the system.tasking.stages.task_wrapper
from /lib64/libgnarl-9.so.  Adding "additional_flags=-bargs
additional_flags=-shared additional_flags=-largs" to the flags argument of
gdb_compile_ada gives us the same PASS, but installing libada7-debuginfo gets
us the same FAIL again.

Fix the FAIL by allowing the 'self_id' argument.

Tested on x86_64-linux.

Change-Id: I5aee5856fa6aeb0cc78aa4fe69deecba5b00b77a