Twelve math and science professors at a Massachusetts Institute of Technology think-tank announced their latest brainstorming success Monday: twenty great holiday gift ideas for the co-worker or loved one who seems to have everything. 'We dedicated ourselves to solving this most universal of problems,' said team leader and biochemistry professor Charles J. Chang, 'and we are proud to say we have come up with 20 great solutions.' Among the ideas: a T-shirt reading, 'It's Not a Beer Gut, It's a Gas Tank For a Sex Machine,' available at Spencer Gifts; a hand-held electronic golf game from The Sharper Image; and a Corvette-shaped videotape rewinder from the Suncoast Motion Picture Company. 'You can rewind your tapes in it,' said team member Dr. Phillip Wasserstein. 'Most people rewind them in their VCRs, but if you have one of these, you won't have to.' 