bug#44674: 28.0.50; Adding current-cpu-time for performance tests

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Mon, 16 Nov 2020 12:46:07 +0100
> Cc: 44674@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>
> 
> > Btw, once this goes in, how about making benchmark-run use it?
> 
> benchmark-run measures walltime, not CPU time

It does now, but we may want to change that.  Does it really make
sense to "benchmark" something using elapsed time?