sim: always call SIM_AC_OPTION_WARNINGS

sim: always call SIM_AC_OPTION_WARNINGS

Now that all ports have opted in to this, we can require it in the
core.  It guarantees that new ports have them turned on, and defaults
to -Werror in the hopes that new ports keep their code clean from the
start.  We do this as a sep commit to make it clear that there are no
changes to existing ports as they've all explicitly called it already.