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

sim: riscv: 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 a new header which only
this port will include.

We can also move the machs.h include out since the model logic was all
generalized from compile-time to runtime last year.