Add ability to run robot main loop in a separate thread (#1895)

Default behavior is still to run the robot main loop in the main thread.

The ability to run the robot main loop in a separate thread and add a hook
for running a different function in the main thread is needed for simulation
GUI support on some platforms.

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

@@ -60,6 +60,7 @@ void InitializeHAL() {
   InitializeExtensions();
   InitializeI2C();
   InitializeInterrupts();
+  InitializeMain();
   InitializeMockHooks();
   InitializeNotifier();
   InitializePDP();