sim: split sim/callback.h include out

sim: split sim/callback.h include out

The sim-basics.h is too big and includes too many things.  This leads
to some arch's sim-main.h having circular loop issues with defs, and
makes it hard to separate out common objects from arch-specific defs.
By splitting up sim-basics.h and killing off sim-main.h, it'll make
it easier to separate out the two.

Start with splitting out sim/callback.h.