sim: invert sim_state storage

Currently all ports have to declare sim_state themselves in their
sim-main.h and then embed the common sim_state_base & sim_cpu in it.
This dynamic makes it impossible to share common object code among
multiple ports because the core data structure is always different.

Let's invert this relationship: common code declares sim_state, and
if the port actually needs state on a per-instance basis, it can use
the new arch_data field for it.  Most ports don't actually use it,
so they don't need to declare anything at all.

This is the first in a series of changes: it adds a define to select
between the old & new layouts, then converts all the ports that don't
need custom state over to the new layout.