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>

core/os/thread.h

@@ -34,6 +34,7 @@
 #include "core/typedefs.h"
 
 #if !defined(NO_THREADS)
+#include "core/templates/safe_refcount.h"
 #include <thread>
 #endif
 
@@ -61,7 +62,7 @@ private:
 	friend class Main;
 
 	static ID main_thread_id;
-	static ID last_thread_id;
+	static SafeNumeric<Thread::ID> last_thread_id;
 
 	ID id = 0;
 	static thread_local ID caller_id;