tools/power turbostat: relax dependency on invariant TSC

Turbostat can be useful on systems that do not support invariant TSC,
so allow it to run on those systgems.

All arithmetic in turbostat using the TSC value is per-processsor,
so it does not depend on the TSC values being in sync acrosss processors.

Turbostat uses gettimeofday() for the measurement interval
rather than using the TSC directly, so that key metric
is also immune from variable TSC.

Turbostat prints a TSC sanity check column:

TSC_MHz = TSC_delta/interval

If this column is constant and is close to the processor
base frequency, then the TSC is behaving properly.

The other key turbostat columns are calculated this way:

Avg_Mhz = APERF_delta/interval

%Busy = MPERF_delta/TSC_delta

Bzy_MHz = TSC_delta/APERF_delta/MPERF_delta/interval

Tested on Core2 and Core2-Xeon, and so this patch includes
a few other changes to remove the assumption that target
systems are Nehalem and newer.

Signed-off-by: Len Brown <len.brown@intel.com>