REORG: clock: move the updates of cpu/mono time to clock.c

REORG: clock: move the updates of cpu/mono time to clock.c

The entering_poll/leaving_poll/measure_idle functions that were hard
to classify and used to move to various locations have now been placed
into clock.c since it's precisely about time-keeping. The functions
were renamed to clock_*. The samp_time and idle_time values are now
static since there is no reason for them to be read from outside.