Re-implement RTC current time calculation to eliminate the possibility of
losing time.

The problem with the earlier implementation was that the uptime value
used by 'vrtc_curtime()' could be different than the uptime value when
'vrtc_time_update()' actually updated 'base_uptime'.

Fix this by calculating and updating the (rtctime, uptime) tuple together.

MFC after: 2 weeks