sim: enable -Werror by default for some arches

sim: enable -Werror by default for some arches

We've had this off for a long time because the sim code was way too
full of warnings for it to be feasible.  However, I've cleaned things
up significantly from when this was first merged, and we can start to
turn this around.

Change the macro to enable -Werror by default, and allow ports to opt
out.  New ports will get it automatically (and we can push back on
them if they try to turn it off).

Also turn it off for the few ports that still hit warnings for me.
All the rest will get the new default, and we'll wait for feedback
if/when new issues come up.