ARM isn't open-source -- it's owned by the company ARM Ltd. You can download and use nnARM for personal-use and academic study. But you would be very foolish to build commercial project around an unauthorized project like nnARM.
There are several open-source MIPS-clones that are good for study. (Probably not so good for actual commercial use, since they are slow, big, and not competive with today's embedded CPUs.)