Is it possible to use wp_localize_script to create global JS variables without a specific script handle?

Instead of using wp_localize_script in that case, you can hook your js variables at wp_head, that way it would be available to all js files
like:

function my_js_variables(){ ?>
      <script type="text/javascript">
        var ajaxurl="<?php echo admin_url( "admin-ajax.php" ); ?>";
        var ajaxnonce="<?php echo wp_create_nonce( "itr_ajax_nonce" ); ?>";
      </script><?php
}
add_action ( 'wp_head', 'my_js_variables' )

Also as suggested by @Weston Ruter, you should json encode the variables:

add_action ( 'wp_head', 'my_js_variables' );
function my_js_variables(){ ?>
  <script type="text/javascript">
    var ajaxurl = <?php echo json_encode( admin_url( "admin-ajax.php" ) ); ?>;      
    var ajaxnonce = <?php echo json_encode( wp_create_nonce( "itr_ajax_nonce" ) ); ?>;
    var myarray = <?php echo json_encode( array( 
         'foo' => 'bar',
         'available' => TRUE,
         'ship' => array( 1, 2, 3, ),
       ) ); ?>
  </script><?php
}