target/s390x: Build 'gdbstub.c' once for system single binary There is a single qemu-system-s390x binary, but by moving 'gdbstub.c' in the target_common_system_arch[] source set the resulting object can be linked into a single qemu-sytem binary. Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org> Message-ID: <20260219191955.83815-49-philmd@linaro.org>