Isolate atomic bit flag for execution.

- Denoted bit_true_atomic only for sys.execute bit settings. All other
bit_true type calls are for local variables only and don’t need atomic
access. Still looking into other ways of setting these flags without
requiring atomic access, but this is a patch for now.

stepper.c

@@ -342,7 +342,7 @@ ISR(TIMER1_COMPA_vect)
     } else {
       // Segment buffer empty. Shutdown.
       st_go_idle();
-      bit_true(sys.execute,EXEC_CYCLE_STOP); // Flag main program for cycle end
+      bit_true_atomic(sys.execute,EXEC_CYCLE_STOP); // Flag main program for cycle end
       return; // Nothing to do but exit.
     }  
   }