sim: iq2000: add fallback for exit syscall

Make sure this syscall always exits regardless of the exit code.

sim/iq2000/iq2000.c

diff --git a/sim/iq2000/iq2000.c b/sim/iq2000/iq2000.c
index 087646b..f35e91e 100644
--- a/sim/iq2000/iq2000.c
+++ b/sim/iq2000/iq2000.c
@@ -77,6 +77,9 @@
 	  /* Fail.  */
 	  puts ("fail");
 	  exit (1);
+	default:
+	  puts ("unknown exit");
+	  exit (2);
 	}
 
     case TARGET_NEWLIB_SYS_write: