board/native: no omit frame pointer if backtrace is used
board/native: no omit frame pointer if backtrace is used

this makes riots test work but may have different result than O0
see man 3 backtrace #NOTES

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