toolchain: gcc: Fix fallthrough attribute for old versions
toolchain: gcc: Fix fallthrough attribute for old versions

fallthrough attribute was introduced in gcc 7. For older versions the
macro is empty to avoid the follow warnning:

~/zephyrproject/zephyr/lib/os/printk.c:268:5: warning: empty declaration
     __fallthrough;

Signed-off-by: Flavio Ceolin <979b9165500b0741b9d0500e2efd74fc1547bff7@intel.com>

nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr