#576 switch to std::_Exit for consistency

- std::_Exit is a little more 'abrupt' than C's exit
  in that it does less flushing and will not call registered
  at-exit's.. I guess, consistency is better? Maybe.

src/vt/runtime/runtime.cc

@@ -136,8 +136,7 @@ Runtime::Runtime(
     }
 
     MPI_Abort(comm, exit_code);
-    exit(exit_code);
-    return;
+    std::_Exit(exit_code); // no return
   }
 
   sig_user_1_ = false;