[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/AccelerometerData.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/AccelerometerData.h"
+
+#include "hal/simulation/SimDataValue.h"
+
+extern "C" {
+void HALSIM_ResetAccelerometerData(int32_t index) {}
+
+#define DEFINE_CAPI(TYPE, CAPINAME, RETURN) \
+  HAL_SIMDATAVALUE_STUB_CAPI(TYPE, HALSIM, Accelerometer##CAPINAME, RETURN)
+
+DEFINE_CAPI(HAL_Bool, Active, false)
+DEFINE_CAPI(HAL_AccelerometerRange, Range, HAL_AccelerometerRange_k2G)
+DEFINE_CAPI(double, X, 0)
+DEFINE_CAPI(double, Y, 0)
+DEFINE_CAPI(double, Z, 0)
+
+void HALSIM_RegisterAccelerometerAllCallbacks(int32_t index,
+                                              HAL_NotifyCallback callback,
+                                              void* param,
+                                              HAL_Bool initialNotify) {}
+}  // extern "C"