Well, one answer was that Stallone, a sometime actor-turned-screenwriter, could sit down and in 3 1/2 days write a screenplay with a meaty starring role in it for himself, persuade someone to film it, and wind up a millionaire. Improbable? Pessimists might say so, and advise Stallone to try something more sure, like the Irish Sweepstakes. Impossible? Well, no, because you see, there's this new movie, called ''Rocky.''