cpu: Fix header include guards

cpu/native/include/clang_compat.h

cpu/nrf51/include/cpu_conf.h

cpu/nrf51/include/nrf51_bitfields.h

cpu/nrf52/include/nrf52_bitfields.h

cpu/sam3/include/cpu_conf.h

cpu/samd21/include/cpu_conf.h

cpu/stellaris_common/include/cortex-m4-def.h

cpu/stellaris_common/include/hw_adc.h

cpu/stellaris_common/include/hw_gpio.h

cpu/stellaris_common/include/hw_hibernate.h

cpu/stellaris_common/include/hw_i2c.h

cpu/stellaris_common/include/hw_ints.h

cpu/stellaris_common/include/hw_memmap.h

cpu/stellaris_common/include/hw_nvic.h

cpu/stellaris_common/include/hw_pwm.h

cpu/stellaris_common/include/hw_ssi.h

cpu/stellaris_common/include/hw_sysctl.h

cpu/stellaris_common/include/hw_sysexc.h

cpu/stellaris_common/include/hw_timer.h

cpu/stellaris_common/include/hw_types.h