sim: or1k: move arch-specific settings to internal header

There's no need for these settings to be in sim-main.h which is shared
with common/ sim code, so move it all out to the existing or1k-sim.h.
Unfortunately, we can't yet drop the or1k-sim.h include from sim-main.h
as many of the generated CGEN files refer only to sim-main.h.  We'll
have to improve the CGEN interface before we can make more progress,
but this is at least a minor improvement.