Don't use the system iconv on Apple mobile platforms

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

(cherry picked from commit d6a48e4958e48e797c946ba231a6d025e636f77c)

CMakeLists.txt

@@ -241,7 +241,7 @@ endif()
 if(UNIX OR MINGW OR MSYS OR (USE_CLANG AND NOT WINDOWS) OR VITA OR PSP OR PS2 OR N3DS)
   set(OPT_DEF_LIBC ON)
 endif()
-if(WINDOWS)
+if(WINDOWS OR IOS OR TVOS)
   set(SDL_SYSTEM_ICONV_DEFAULT OFF)
 else()
   set(SDL_SYSTEM_ICONV_DEFAULT ON)