sim: sh: fix -Wimplicit-fallthrough warnings

These generate conditional insns where it tests, then fallsthru.
diff --git a/sim/sh/gencode.c b/sim/sh/gencode.c
index 1835cf9..9683dc4 100644
--- a/sim/sh/gencode.c
+++ b/sim/sh/gencode.c
@@ -3296,6 +3296,7 @@
 	    case 'c':
 	      printf ("      if ((((iword >> 8) ^ DSR) & 1) == 0)\n");
 	      printf ("\treturn;\n");
+	      printf ("      ATTRIBUTE_FALLTHROUGH;\n");
 	      printf ("    }\n");
 	      printf ("  case %d:\n", p->index + 1);
 	      printf ("    {\n");