app/flow-perf: change clock measurement

app/flow-perf: change clock measurement

The clock() function is not good practice to use for multiple
cores/threads, since it measures the CPU time used by the process
and not the wall clock time, while when running through multiple
cores/threads simultaneously, we can burn through CPU time much
faster.

As a result this commit will change the way of measurement to use
rd_tsc, and the results will be divided by the processor frequency.

Signed-off-by: Wisam Jaddo <wisamm@nvidia.com>
Reviewed-by: Alexander Kozyrev <akozyrev@nvidia.com>
Reviewed-by: Suanming Mou <suanmingm@nvidia.com>