[hal] Add high level way of getting systemserver NT instance (#7683)

hal/src/main/native/sim/SystemServer.cpp

@@ -0,0 +1,12 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "hal/SystemServer.h"
+
+extern "C" {
+
+NT_Inst HAL_GetSystemServerHandle(void) {
+  return 0;
+}
+}  // extern "C"