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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Mon, 16 Nov 2020 11:53:37 +0100
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 44674@debbugs.gnu.org,
>         Philipp Stephani <p.stephani2@gmail.com>
> 
> 16 nov. 2020 kl. 11.40 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > AFAUI, CLOCK_PROCESS_CPUTIME_ID accounts for all the threads, so might not 
> > be what we want even if it is supported, since other threads might be 
> > involved which we aren't interested in (e.g., GTK threads).
> 
> CLOCK_THREAD_CPUTIME_ID then. Actually, a case can be made for either timer.

Can you make a case for process-global timing, given that we don't
control all of the threads, and that some of them are utterly
unrelated to timing Lisp programs?