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/nrun.c

@@ -23,6 +23,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 #include <string.h>
 
 #include "sim-main.h"
+#include "sim-signal.h"
 #include "sim/callback.h"
 
 #include "bfd.h"