sim: move default model to the runtime sim state

sim: move default model to the runtime sim state

This kills off another compile-time option by moving the setting to
the individual arch runtimes.  This will allow dynamic selection by
the arch when doing a single build with multiple arches.

The sim_model_init rework is a little funky.  In the past it was
disabled entirely if no default model was set.  We maintain the
spirit of the logic by gating the fallback logic on whether the
port has defined any models.