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.

sim/common/sim-basics.h

@@ -112,7 +112,6 @@ typedef enum {
 /* Basic definitions - ordered so that nothing calls what comes after it.  */
 
 #include "ansidecl.h"
-#include "sim/callback.h"
 #include "sim/sim.h"
 
 #include "sim-config.h"