SUPPORTING HER EX:Breathy London pop star Jane Birkin will sultrily sing to honor the great French pop singer/ troublemaker Serge Gainsbourg, her one-time husband and collaborator, tomorrow night at 8:30. Birkin and Gainsbourg’s sex- dosed song “Je T’Aime . . . Moi Non Plus” was banned on radio but managed to be a No. 1 hit throughout Europe in 1969. Birkin and the group Djam & Fam of North Africa will pepper the tunes with Moroccan and Algerian rhythms. Tick ets: $40 if purchased today, $45 tomorrow. Skirball Center, 566 La Guardia Place, on the NYU Campus; (212) 992-8484.