tests/periph_rtc: fix system locks in ISR
tests/periph_rtc: fix system locks in ISR

`tests/periph` calls the functions `rtc_get_alarm` and `rtc_set_alarm` from an ISR. Depending on the implementation of `periph_rtc`, however, these functions use `newlib` functions such as `localtime` and `mktime`, which in turn call the `newlib` lock function `__tz_lock`. This can lead to a system lock if the `newlib`  with real lock functionality is used instead of the `newlib_nano`.
ant9000/RIOT,yogo1212/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,smlng/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,OTAkeys/RIOT,smlng/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,yogo1212/RIOT,kYc0o/RIOT,OTAkeys/RIOT,ant9000/RIOT,basilfx/RIOT,RIOT-OS/RIOT,basilfx/RIOT,basilfx/RIOT,kaspar030/RIOT,basilfx/RIOT,RIOT-OS/RIOT,smlng/RIOT,basilfx/RIOT,OTAkeys/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,smlng/RIOT,yogo1212/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,smlng/RIOT,authmillenon/RIOT,OlegHahm/RIOT,authmillenon/RIOT,yogo1212/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT