By using the RTC helper functions instead of POSIX mktime()/gmtime()
we can not only extend the RTC range beyond Y2038.
For tests/periph_rtc:
before:
text data bss dec hex filename
28028 248 2472 30748 781c stk3700/tests_periph_rtc.elf
after:
text data bss dec hex filename
19400 144 2424 21968 55d0 stk3700/tests_periph_rtc.elf
fixes#13277