cpu/cortexm_common: fix r12 clobber in pendsv for Cortex-M3+
cpu/cortexm_common: fix r12 clobber in pendsv for Cortex-M3+

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