sim: cris: fix -Wimplicit-fallthrough warnings

Replace some fall through comments with the attribute.

sim/cris/cris-tmpl.c

@@ -67,6 +67,10 @@ MY (f_break_handler) (SIM_CPU *cpu, USI breaknum, USI pc)
       cris_break_13_handler (cpu, /* TARGET_SYS_exit */ 1, 0,
 			     0, 0, 0, 0, 0, pc);
 
+      /* This shouldn't be reached, but we can't mark break 13 as noreturn
+	 since there are some calls which should return.  */
+      ATTRIBUTE_FALLTHROUGH;
+
     default:
       abort ();
     }

sim/cris/traps.c

@@ -1527,7 +1527,7 @@ cris_break_13_handler (SIM_CPU *current_cpu, USI callnum, USI arg1,
 		  retval = TARGET_O_WRONLY;
 		  break;
 		}
-	      /* FALLTHROUGH */
+	      ATTRIBUTE_FALLTHROUGH;
 	    default:
 	      /* Nothing else is implemented.  */
 	      retval