Use linux-perf to access the hardware instructions-retired counter.
This is necessary because the counter is not enabled by default,
and also the counter is prone to roll-over -- both of which
perf manages.

It is not necessary to use perf for the cycle counter,
because turbostat already needs to collect delta-aperf
to calcuate frequency.
Signed-off-by: Len Brown <len.brown@intel.com>