Use RTC helper functions instead of libc functions.
This gives us y2038 safety by the extended epoch and saves
a good chunk of memory:
picolibc mktime():
text data bss dec hex filename
15048 520 2504 18072 4698 tests/periph_rtc/bin/hifive1/tests_periph_rtc.elf
rtc_mktime():
text data bss dec hex filename
7632 40 2452 10124 278c tests/periph_rtc/bin/hifive1/tests_periph_rtc.elf