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/example-synacor/sim-main.c

@@ -25,6 +25,7 @@
 #include "defs.h"
 
 #include "sim-main.h"
+#include "sim-signal.h"
 
 /* Get the register number from the number.  */
 static unsigned16