Fannie Mae and Freddie Mac, facing the advent of tougher affordable housing goals in the coming year, are serving notice that they both plan to move more aggressively into the subprime mortgage arena as a way to tap lower income borrowers and targeted markets. Speaking at the recent annual conventions of both America’s Community Bankers and the Mortgage Bankers Association, top executives at the GSEs announced their intention to move down the mortgage borrower credit…