Recalibrate TSC timecounter with HPET and PM timer

If frequency of an invariant (non-stop) time stamp counter is measured
using an independent working timecounter that has a known frequency, we
can assume that the measured TSC frequency is as good as the resolution
of the timecounter that we use to perform the measurement. This lets us
switch from this high quality but expensive source to the cheaper TSC
without sacrificing precision on a wide range of modern CPUs.

From Adam Steen <adam@adamsteen.com.au> with tweaks from reyk@ and myself.

Tested by brynet@, sthen@ and others, OK mlarkin, sthen

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