cortexm_common: simplify FPU guards
cortexm_common: simplify FPU guards

The FPU is only enabled on platforms where it is available. The extra
checks for these platforms are redundant and removed here

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