WORKERS SECURED the site of an excavation cave-in near the White House Wednesday, allowing United Press International to move back into its headquarters, as crews labored to repair severed telephone and power lines . . . PHILIPPINE President Corazon Aquino vowed Thursday to ease the tax load for low-income earners and improve their social security benefits in a bid to minimize the impact of the Persian Gulf crisis and defuse unrest among workers demanding a wage increase . . . CHINA HAS STOPPED supplying weapons to Cambodian guerrilla forces, a Chinese foreign ministry spokesman said Thursday. The statement by Li Zhaoxing was the first public pronouncement by China that it was no longer arming the Khmer Rouge, diplomats said.