Once again, Michael J. Fox was the chief executive at the nation's movie houses over the weekend. Universal's "Secret of My Success," Fox's latest, took in $5.6 million at 1,346 venues; the film has so far grossed $27.2 million since its April 10 release. Tri-Star's shoot-'em-up neo-Western, "Extreme Prejudice," was the class of the debutantes, loading up with $3.5 million at 1,071 screens its first weekend. This week's tidbit: Touchstone's "Outrageous Fortune" and Warners' "Lethal Weapon" were the first two films released in 1987 to crack the $50-million mark (the sure sign of a hit). "Outrageous" did the trick in 11 weeks, while it took "Weapon" only six.