Tony Award winner, former Spider-Man — to tap into his inner Johnny Knoxville? Have the real guy on hand, said director Gia Coppola.The “Jackass” series creator plays a small role in Coppola’s new movie, “Mainstream,” out Friday, which stars Garfield as a charismatic drifter who becomes an internet sensation. For some of the film’s biggest scenes, Coppola tapped Knoxville’s pranking advice.