Submit callback; run cron.
function system_run_cron_submit($form, &$form_state) {
// Run cron manually from Cron form.
if (drupal_cron_run()) {
drupal_set_message(t('Cron run successfully.'));
}
else {
drupal_set_message(t('Cron run failed.'), 'error');
}
drupal_goto('admin/config/system/cron');
}