cw: don't shadow pure pre-processing

@@ -1469,11 +1469,13 @@
         default:
                 (void) fprintf(stderr,
                     "Incompatible -xarch= and/or -m32/-m64 options used.\n");
                 exit(2);
         }
-        if (op == CW_O_LINK && (ctx->i_flags & CW_F_SHADOW))
+
+        if ((op == CW_O_LINK || op == CW_O_PREPROCESS) &&
+            (ctx->i_flags & CW_F_SHADOW))
                 exit(0);
 
         if (model && !pic)
                 newae(ctx->i_ae, model);
         if (!nolibc)