Bethesda-based Lockheed Martin said Tuesday that it is offering a voluntary layoff program for about 6,500 U.S.-based employees, the latest in a string of recent moves to cut jobs at the company. The news comes as the Pentagon continues to push for savings from contractors. The initiative offers a severance package to all U.S.-based, salaried employees who report to Lockheed’s corporate headquarters or internal business services organization.