sim: mn10300: standardize the arch-specific settings a little

Rename mn10300_sim.h to mn10300-sim.h to match other ports, and move most
of the arch-specific content out of sim-main.h to it.  This isn't a big
win though as we still have to include the header in sim-main.h due to the
igen interface: it hardcodes including sim-main.h in its files.  So until
we can fix that, we have to keep bleeding these settings into the common
codes.

Also take the opportunity to purge a lot of unused headers from these.
The local modules should already include the right headers, so there's
no need to force everyone to pull them in.  A lot of this is a hold over
from the pre-igen days of this port.