sim: namespace sim_machs

sim: namespace sim_machs

We want to do a single build with all arches in one binary which means
we need to namespace sim_machs on a per-arch basis.  Move it from a
global variable to the sim description structure so it can be setup at
runtime.

Changing the SIM_MODEL->num from an enum to an int is unfortunate, but
we specifically don't want to maintain a centralized list anymore, and
this was never used directly in common code, just passed to per-arch
callbacks.