Some Macs already say they’re ‘‘Assembled in USA.’’ That’s because Apple has for years performed final assembly of some units in the U.S. Those machines are usually the product of special orders placed at its online store. The last step of production may consist of mounting hard drives, memory chips and graphics cards into computer cases that are manufactured elsewhere. With Cook’s announcement Thursday, the company is set to go much further in the amount of work done in the U.S.