rtc: factor out RTC_Linux_ReadTimeAfterInterrupt

rtc: factor out RTC_Linux_ReadTimeAfterInterrupt

We have code to read time after an RTC's UIE interrupt, which we
currently use as part of maintaining the correction file.

In a later commit, we will need the same functionality for using the RTC
as reference clock, so export the function and give it a descriptive
name appropriate for a globally visible function.