Copied 'contiki_arg[c|v]' from 'native' to the other desktop targets.

- On the retro targets support for 'contiki_arg[c|v]' is optional as accesing arg[c|v] is rather expensive.
Merge pull request #536 from oliverschmidt/master

Copied 'contiki_arg[c|v]' from 'native' to the other desktop targets.