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/cpp/Main.cpp

@@ -0,0 +1,64 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2019 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/Main.h"
+
+#include <wpi/condition_variable.h>
+#include <wpi/mutex.h>
+
+static void DefaultMain(void*);
+static void DefaultExit(void*);
+
+static bool gHasMain = false;
+static void* gMainParam = nullptr;
+static void (*gMainFunc)(void*) = DefaultMain;
+static void (*gExitFunc)(void*) = DefaultExit;
+static bool gExited = false;
+struct MainObj {
+  wpi::mutex gExitMutex;
+  wpi::condition_variable gExitCv;
+};
+
+static MainObj* mainObj;
+
+static void DefaultMain(void*) {
+  std::unique_lock lock{mainObj->gExitMutex};
+  mainObj->gExitCv.wait(lock, [] { return gExited; });
+}
+
+static void DefaultExit(void*) {
+  std::lock_guard lock{mainObj->gExitMutex};
+  gExited = true;
+  mainObj->gExitCv.notify_all();
+}
+
+namespace hal {
+namespace init {
+void InitializeMain() {
+  static MainObj mO;
+  mainObj = &mO;
+}
+}  // namespace init
+}  // namespace hal
+
+extern "C" {
+
+void HAL_SetMain(void* param, void (*mainFunc)(void*),
+                 void (*exitFunc)(void*)) {
+  gHasMain = true;
+  gMainParam = param;
+  gMainFunc = mainFunc;
+  gExitFunc = exitFunc;
+}
+
+HAL_Bool HAL_HasMain(void) { return gHasMain; }
+
+void HAL_RunMain(void) { gMainFunc(gMainParam); }
+
+void HAL_ExitMain(void) { gExitFunc(gMainParam); }
+
+}  // extern "C"