[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/DIOData.cpp

@@ -0,0 +1,28 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017-2020 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/DIOData.h"
+
+#include "hal/simulation/SimDataValue.h"
+
+extern "C" {
+void HALSIM_ResetDIOData(int32_t index) {}
+
+HAL_SimDeviceHandle HALSIM_GetDIOSimDevice(int32_t index) { return 0; }
+
+#define DEFINE_CAPI(TYPE, CAPINAME, RETURN) \
+  HAL_SIMDATAVALUE_STUB_CAPI(TYPE, HALSIM, DIO##CAPINAME, RETURN)
+
+DEFINE_CAPI(HAL_Bool, Initialized, false)
+DEFINE_CAPI(HAL_Bool, Value, false)
+DEFINE_CAPI(double, PulseLength, 0)
+DEFINE_CAPI(HAL_Bool, IsInput, false)
+DEFINE_CAPI(int32_t, FilterIndex, 0)
+
+void HALSIM_RegisterDIOAllCallbacks(int32_t index, HAL_NotifyCallback callback,
+                                    void* param, HAL_Bool initialNotify) {}
+}  // extern "C"