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/core_bind.h

@@ -40,6 +40,7 @@
 #include "core/os/os.h"
 #include "core/os/semaphore.h"
 #include "core/os/thread.h"
+#include "core/templates/safe_refcount.h"
 
 class _ResourceLoader : public Object {
 	GDCLASS(_ResourceLoader, Object);
@@ -559,7 +560,7 @@ class _Thread : public Reference {
 protected:
 	Variant ret;
 	Variant userdata;
-	volatile bool active = false;
+	SafeFlag active;
 	Object *target_instance = nullptr;
 	StringName target_method;
 	Thread thread;