[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/CanDataInternal.cpp

@@ -0,0 +1,25 @@
+/*----------------------------------------------------------------------------*/
+/* 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/CanData.h"
+#include "hal/simulation/SimDataValue.h"
+
+extern "C" {
+
+void HALSIM_ResetCanData(void) {}
+
+#define DEFINE_CAPI(TYPE, CAPINAME) \
+  HAL_SIMCALLBACKREGISTRY_STUB_CAPI_NOINDEX(TYPE, HALSIM, Can##CAPINAME)
+
+DEFINE_CAPI(HAL_CAN_SendMessageCallback, SendMessage)
+DEFINE_CAPI(HAL_CAN_ReceiveMessageCallback, ReceiveMessage)
+DEFINE_CAPI(HAL_CAN_OpenStreamSessionCallback, OpenStream)
+DEFINE_CAPI(HAL_CAN_CloseStreamSessionCallback, CloseStream)
+DEFINE_CAPI(HAL_CAN_ReadStreamSessionCallback, ReadStream)
+DEFINE_CAPI(HAL_CAN_GetCANStatusCallback, GetCANStatus)
+
+}  // extern "C"