sim: split sim-signal.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.

sim/common/sim-basics.h

@@ -121,7 +121,6 @@ typedef enum {
 #include "sim-types.h"
 #include "sim-bits.h"
 #include "sim-endian.h"
-#include "sim-signal.h"
 
 #include "sim-utils.h"