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-signal.c

@@ -22,6 +22,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <signal.h>
 #include "sim-main.h"
+#include "sim-signal.h"
 
 /* Convert SIM_SIGFOO to SIGFOO.
    What to do when the host doesn't have SIGFOO is handled on a case by case