Having a monopoly or being in a dominant position is not the problem. If ARM starts a pizza business and requires everyone who wants to buy mobile CPUs to also order pizzas exclusively from them, then this would be an equally-fineable abuse of a dominant market position and the EU would almost certainly step in.
As it stands, ARM doesn’t force others to buy pizzas exclusively from ARM PIZZA PLACE and hence doesn’t abuse its dominant market position in one market (mobile CPUs) to support its position in another market (pizzas).
Google, on the other hand, uses its dominant position in the "licensable mobile operating systems" market to support its position in the "internet search" and "browsers" market together with anticompetitive behaviour forcing its licensees to exclusively use the Google-approved version of Android.