sys/architecture: Add UWORD_MIN, UWORD_MAX, ...
sys/architecture: Add UWORD_MIN, UWORD_MAX, ...

Allow accessing the highest and lowest number an uword_t and an sword_t can hold
via a preprocessor macro with a speaking name.

authmillenon/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OTAkeys/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,authmillenon/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT