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` by the new `SafeFlag`
- Platform-specific implementations no longer needed

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

scene/main/http_request.h

@@ -34,6 +34,7 @@
 #include "core/io/http_client.h"
 #include "core/os/file_access.h"
 #include "core/os/thread.h"
+#include "core/safe_refcount.h"
 #include "node.h"
 #include "scene/main/timer.h"
 
@@ -74,7 +75,7 @@ private:
 	bool request_sent;
 	Ref<HTTPClient> client;
 	PoolByteArray body;
-	volatile bool use_threads;
+	SafeFlag use_threads;
 
 	bool got_response;
 	int response_code;
@@ -85,7 +86,7 @@ private:
 	FileAccess *file;
 
 	int body_len;
-	volatile int downloaded;
+	SafeNumeric<int> downloaded;
 	int body_size_limit;
 
 	int redirections;
@@ -103,8 +104,8 @@ private:
 	Error _parse_url(const String &p_url);
 	Error _request();
 
-	volatile bool thread_done;
-	volatile bool thread_request_quit;
+	SafeFlag thread_done;
+	SafeFlag thread_request_quit;
 
 	Thread thread;