Fix a behavioral change accidentally introduced by the move to the LLVM

commandline system: `-output-(bc|ll|s)` used to disable generation of an object
file unless explicitly requested with `-output-o`. Now they do so again.

gen/main.cpp

@@ -238,7 +238,8 @@ int main(int argc, char** argv)
         VersionCondition::addGlobalIdent);
 
     global.params.output_o =
-        opts::output_o == cl::BOU_UNSET
+        (opts::output_o == cl::BOU_UNSET
+            && !(opts::output_bc || opts::output_ll || opts::output_s))
         ? OUTPUTFLAGdefault
         : opts::output_o == cl::BOU_TRUE
             ? OUTPUTFLAGset