Wordpress - 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
}

You can export any data you want in the wp_head hook, as the answers above show. However, you should use json_encode to prepare the PHP data for exporting to JS instead of trying to embed raw values into JS literals:

function my_js_variables(){
    ?>
    <script>
    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( 
        'food' => 'bard',
        'bard' => false,
        'quux' => array( 1, 2, 3, ),
    ) ) ?>;
    </script>
    <?php
}
add_action ( 'wp_head', 'my_js_variables' )

Using json_encode makes it easier on yourself, and it prevents accidental syntax errors if your string includes any quote marks. Even more importantly, using json_encode thwarts XSS attacks.