rtc: export RTC_Linux_{SwitchInterrupt,CheckInterrupt}

rtc: export RTC_Linux_{SwitchInterrupt,CheckInterrupt}

We have code to enable and disable the RTC's UIE interrupt, as well as
check whether it occurred and skip the first interrupt as it may be
bogus.
This is currently used 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 functions and give them descriptive
names appropriate for globally visible functions.