Windows GDK Support (#5830)

* Added GDK

* Simplfied checks in SDL_config_wingdk.h

* Added testgdk sample

* Added GDK readme

* Fixed error in merge of SDL_windows.h

* Additional GDK fixes

* OpenWatcom should not export _SDL_GDKGetTaskQueue

* Formatting fixes

* Moved initialization code into SDL_GDKRunApp

src/atomic/SDL_spinlock.c

@@ -20,7 +20,7 @@
 */
 #include "../SDL_internal.h"
 
-#if defined(__WIN32__) || defined(__WINRT__)
+#if defined(__WIN32__) || defined(__WINRT__) || defined(__GDK__)
 #include "../core/windows/SDL_windows.h"
 #endif