sim: v850: standardize the arch-specific settings a little

Rename v850_sim.h to v850-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.