Don't use the system iconv on Apple mobile platforms

Fixes https://github.com/libsdl-org/SDL/issues/8964

CMakeLists.txt

@@ -153,7 +153,7 @@ endif()
 # Default option knobs
 set(SDL_LIBC_DEFAULT ON)
 set(SDL_SYSTEM_ICONV_DEFAULT ON)
-if(WINDOWS)
+if(WINDOWS OR IOS OR TVOS OR VISIONOS)
   set(SDL_SYSTEM_ICONV_DEFAULT OFF)
 endif()