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/object/object.cpp

@@ -1727,7 +1727,7 @@ void *Object::get_script_instance_binding(int p_script_language_index) {
 	if (!_script_instance_bindings[p_script_language_index]) {
 		void *script_data = ScriptServer::get_language(p_script_language_index)->alloc_instance_binding_data(this);
 		if (script_data) {
-			atomic_increment(&instance_binding_count);
+			instance_binding_count.increment();
 			_script_instance_bindings[p_script_language_index] = script_data;
 		}
 	}