[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/PDPData.cpp

@@ -0,0 +1,34 @@
+/*----------------------------------------------------------------------------*/
+/* 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/PDPData.h"
+
+#include "../PortsInternal.h"
+#include "hal/simulation/SimDataValue.h"
+
+extern "C" {
+void HALSIM_ResetPDPData(int32_t index) {}
+
+#define DEFINE_CAPI(TYPE, CAPINAME, RETURN) \
+  HAL_SIMDATAVALUE_STUB_CAPI(TYPE, HALSIM, PDP##CAPINAME, RETURN)
+
+DEFINE_CAPI(HAL_Bool, Initialized, false)
+DEFINE_CAPI(double, Temperature, 0)
+DEFINE_CAPI(double, Voltage, 0)
+HAL_SIMDATAVALUE_STUB_CAPI_CHANNEL(double, HALSIM, PDPCurrent, 0)
+
+void HALSIM_GetPDPAllCurrents(int32_t index, double* currents) {
+  for (int i = 0; i < hal::kNumPDPChannels; i++) currents[i] = 0;
+}
+
+void HALSIM_SetPDPAllCurrents(int32_t index, const double* currents) {}
+
+void HALSIM_RegisterPDPAllNonCurrentCallbacks(int32_t index, int32_t channel,
+                                              HAL_NotifyCallback callback,
+                                              void* param,
+                                              HAL_Bool initialNotify) {}
+}  // extern "C"