cpu/arm7_common: add __ARM_FEATURE_CLZ feature test
cpu/arm7_common: add __ARM_FEATURE_CLZ feature test

The feature test macro should work on all ARM CPUs supported by gcc.

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