WASHINGTON — Goldman Sachs, whose tactics exiting the collapsing subprime mortgage market have been under government scrutiny for months, now faces federal fraud charges that it duped investors into losing $1 billion on a rigged offshore deal pegged to dicey home loans. The suit, brought Friday by the Securities and Exchange Commission, accuses Goldman and one of its vice presidents, 31-year-old Fabrice Tourre, of allowing a Wall Street hedge fund to secretly select many of the securities in the deal.