Use Apple's nomenclature for macOS and iOS

Fixes https://github.com/libsdl-org/SDL/issues/6621

test/checkkeysthreads.c

@@ -254,7 +254,7 @@ main(int argc, char *argv[])
     renderer = SDL_CreateRenderer(window, -1, 0);
     SDL_RenderPresent(renderer);
 
-#if __IPHONEOS__
+#if __IOS__
     /* Creating the context creates the view, which we need to show keyboard */
     SDL_GL_CreateContext(window);
 #endif