system: Reduce 'arch_init.h' scope

Better to restrict qemu_arch_available() scope so it is only used by generic system/ code to parse command line options. Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org> Reviewed-by: Thomas Huth <thuth@redhat.com> Message-Id: <20260213175032.32121-6-philmd@linaro.org>