The Los Angeles-based thrift holding company, the nation's second-largest, said it will acquire small, privately held Tucson Savings & Loan Assn. for an undisclosed amount of Ahmanson stock. Ahmanson, parent of Home Savings of America, is expected to use the acquisition as a springboard to build a presence in Arizona, where it now operates only two lending offices. The deal is subject to approvals by regulators and Tucson Savings shareholders. The Arizona thrift, which operates one branch in Tucson, has assets of $40 million and net worth of $3.1 million.