sim: aarch64: 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.

While we're here, drop redundant includes from sim-main.h:
* sim-types.h is included by sim-base.h already
* sim-base.h is included twice
* sim-io.h is included by sim-base.h already