Lou Rawls, Friday, April 21, at the Odgen Theater, has been singing for over thirty years, and during that time he's mastered an impressive variety of styles: gospel, R&B, Philly soul, smooth-talking pre-rap, blues, jazz and crossover pop. He got his start at Chicago's Dunbar High School, where he polished his doo-wop chops in the lavatory with classmate Sam Cooke. By the end of the Fifties, with high school and a stint in the army behind him, Rawls got his first taste of fame in the gospel act the Pilgrim Travelers. Shortly thereafter he established himself as a secular solo artist with "Tobacco Road," "Stormy Monday," "A Natural Man" and "You'll Never Find Another Love Like Mine"--numbers that made his voice among the more recognizable sounds on the charts. Times have been leaner recently; Rawls is probably best known today for providing the singing voice for Garfield, the animated feline. Fortunately, concert audiences understand that he remains one hip cat.