DisableStartThread is defined for Java threads

  develop(bool, DisableStartThread, false, \
          "Disable starting of additional Java threads " \
          "(for debugging only)")

but is being applied to GC worker threads. For example
share/vm/gc/shared/workgroup.cpp

 64 if (!DisableStartThread) {
 65 os::start_thread(new_worker);
 66 }