While Mayor Thomas Menino and his rival Michael Flaherty, a City Councillor At-Large, will be battering away at each other at the JFK Library, here's where their supporters will be: After a standout, Menino supporters will head over to the headquarters of 1199 SEIU United Healthcare Workers East at 150 Mt. Vernon St. in Dorchester (and right above the offices of the Dorchester Reporter). Flaherty backers will be over at the Banshee at 934 Dorchester Ave., an old haunt of UMass-Boston's student newspaper, the Mass Media. The 7 p.m. debate is sponsored by the Boston Globe, New England Cable News, WGBH-TV and WBUR-FM radio. NECN anchor R.D. Sahl and WGBH's Greater Boston host Emily Rooney will moderate.