PHILADELPHIA— Jimmy Fallon, sportscasters throughout the country and op-ed columnists can stop making them the butt of jokes and wisecracks now. The Philadelphia 76ers will only share a piece of the record book, rather than having it all to themselves. By scoring 70 first-half points on their way to a 123-98 blowout of the totally disinterested Detroit Pistons here last night, the Sixers avoided the ignominy of owning professional sports’ all-time losing streak. Never will a team be so willing and grateful