[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/cpp/jni/simulation/SimulatorJNI.h

@@ -0,0 +1,24 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018-2020 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "hal/Types.h"
+#include "jni.h"
+
+typedef HAL_Handle SIM_JniHandle;
+
+namespace hal {
+namespace sim {
+JavaVM* GetJVM();
+
+jmethodID GetNotifyCallback();
+jmethodID GetBufferCallback();
+jmethodID GetConstBufferCallback();
+jmethodID GetSpiReadAutoReceiveBufferCallback();
+}  // namespace sim
+}  // namespace hal