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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  44674@debbugs.gnu.org,  Philipp Stephani
>  <p.stephani2@gmail.com>
> Date: Mon, 16 Nov 2020 10:27:29 -0500
> 
> > AFAIU, using 'clock' here is not the best idea, as there are caveats wrt to
> > calling 'system', and the origin of the returned value is not well defined
> > to be portable.
> 
> What do you mean by "origin" and by "calling 'system'"?

Quoting https://pubs.opengroup.org/onlinepubs/9699919799/:

          The clock() function shall return the implementation's best
          approximation to the processor time used by the process
          since the beginning of an implementation-defined era
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          related only to the process invocation.

How do you write portable Lisp code that returns consistent results
based on such shaky foundations?