[hal] Implement stub mockdata on RoboRIO (#2575)

This makes it much more user-friendly to use simulation classes without needing
to ifdef for C++ to avoid linker errors or be very careful about construction
to avoid runtime errors in Java.

hal/src/main/native/athena/mockdata/NotifierData.cpp

@@ -0,0 +1,20 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2016-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "hal/simulation/NotifierData.h"
+
+extern "C" {
+
+uint64_t HALSIM_GetNextNotifierTimeout(void) { return 0; }
+
+int32_t HALSIM_GetNumNotifiers(void) { return 0; }
+
+int32_t HALSIM_GetNotifierInfo(struct HALSIM_NotifierInfo* arr, int32_t size) {
+  return 0;
+}
+
+}  // extern "C"