Anti-apartheid activism has replaced anti-nuke consciousness as the cause of choice for rock musicians. In 1986, Little Steven Van Zandt, Bruce Springsteen, U2's Bono Hewson and others recorded "Sun City." This year, such events abound. Paul Simon's "Graceland" Tour, wrapping in New York July 2, will donate proceeds from its Madison Square Garden concert to Children of Apartheid and the United Negro College Fund. In Los Angeles on Sept. 6, black pop and jazz musicians Jean Carn, Roy Ayers, Noel Pointer and the funk band Mtume will stage the "Bishop Tutu Peace Concert" to benefit the African Bursary Fund, which subsidizes South African children's education.