base-nova: workaround for compiling with GCC 6

Issue #2378

repos/base-nova/src/lib/base/stack.cc

@@ -113,6 +113,8 @@ void prepare_reinit_main_thread()
  ** Thread **
  ************/
 
+/* prevent the compiler from optimizing out the 'this' pointer check */
+__attribute__((optimize("-fno-delete-null-pointer-checks")))
 Native_utcb *Thread::utcb()
 {
 	/*