gdbstub: specialise handle_query_attached

In both user and softmmu cases we are just replying with a constant.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20230302190846.2593720-15-alex.bennee@linaro.org>
Message-Id: <20230303025805.625589-15-richard.henderson@linaro.org>