By implementing hook_link_alter() in either a custom module or your theme, the (untested) code would be similar to the following one.

/**
 * Implements hook_link_alter().
 */
function mymodule_link_alter(&$variables) {
  if (isset($variables['options']['attributes']['target']) && $variables['options']['attributes']['target'] == '_blank') {
    $variables['options']['attributes']['rel'] = 'noopener noreferrer';
  }
}

By implementing hook_link_alter in either a custom module or your theme.

The code (untested) would be similar to:

/**
 * Implements hook_link_alter().
 */
function MODULE/THEME_link_alter(&$variables) {
  if (isset($variables['options']['attributes']['target']) && $variables['options']['attributes']['target'] == '_blank') {
    $variables['options']['attributes']['rel'] = 'noopener noreferrer';
  }
}

By implementing hook_link_alter in either a custom module or your theme.

The code (untested) would be similar to:

/**
 * Implements hook_link_alter().
 */
function MODULE/THEME_link_alter(&$variables) {
  if (isset($variables['options']['attributes']['target']) && $variables['options']['attributes']['target'] == '_blank') {
    $variables['options']['attributes']['rel'] = 'noopener noreferrer';
  }
}

By implementing hook_link_alter() in either a custom module or your theme, the (untested) code would be similar to the following one.

/**
 * Implements hook_link_alter().
 */
function mymodule_link_alter(&$variables) {
  if (isset($variables['options']['attributes']['target']) && $variables['options']['attributes']['target'] == '_blank') {
    $variables['options']['attributes']['rel'] = 'noopener noreferrer';
  }
}