Modernize atomics

- Based on C++11's `atomic`
- Reworked `SafeRefCount` (based on the rewrite by @hpvb)
- Replaced free atomic functions by the new `SafeNumeric<T>`
- Replaced wrong cases of `volatile bool` by the new `SafeFlag`
- Platform-specific implementations no longer needed

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>

platform/linuxbsd/joypad_linux.h

@@ -73,7 +73,7 @@ private:
 #ifdef UDEV_ENABLED
 	bool use_udev = false;
 #endif
-	bool exit_monitor = false;
+	SafeFlag exit_monitor;
 	Mutex joy_mutex;
 	Thread joy_thread;
 	Input *input = nullptr;