XANADUHollywood never learns. Hot on the heels of box office failures Sgt. Pepper's Lonely Hearts Club Band and Can't Stop the Music, comes the roller skating Olivia Newton John in Xanadu. This soundtrack is fluff stuff to be sure, but some pearls float amongst the mire. Lead off "Magic" remains a fine single. "Suddenly," a duet ONJ ekes out with British luminary Cliff Richard, seems better than most love themes. The sudden appearance of the Tubes almost