PortNintendo 3DS SDL_main to header-only + SDL_N3DSRunApp()

and move the #undef main and #define main SDL_main to the start/end of
SDL_main_impl.h instead of repeating it in every platform implementation

Thanks to SDL_N3DSRunApp we don't need the #include <3ds.h> in
SDL_main_impl.h - that caused conflicts with testthread.c, because both
have (different) ThreadFunc typedefs.

CMakeLists.txt

@@ -2688,6 +2688,9 @@ elseif(N3DS)
   file(GLOB N3DS_MAIN_SOURCES ${SDL3_SOURCE_DIR}/src/main/n3ds/*.c)
   set(SDLMAIN_SOURCES ${SDLMAIN_SOURCES} ${N3DS_MAIN_SOURCES})
 
+  file(GLOB N3DS_CORE_SOURCES ${SDL3_SOURCE_DIR}/src/core/n3ds/*.c)
+  list(APPEND SOURCE_FILES ${N3DS_CORE_SOURCES})
+
   if(SDL_AUDIO)
     set(SDL_AUDIO_DRIVER_N3DS 1)
     file(GLOB N3DS_AUDIO_SOURCES ${SDL3_SOURCE_DIR}/src/audio/n3ds/*.c)