Use Apple's nomenclature for macOS and iOS

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

test/checkkeys.c

@@ -275,7 +275,7 @@ main(int argc, char *argv[])
 
     textwin = SDLTest_TextWindowCreate(0, 0, 640, 480);
 
-#if __IPHONEOS__
+#if __IOS__
     /* Creating the context creates the view, which we need to show keyboard */
     SDL_GL_CreateContext(window);
 #endif