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

[Mattias Engdegård]

> +The return value is a pair (CPU-TICKS . TICKS-PER-SEC).

Perhaps not ideal to cons in a timing primitive where low overhead is called 
for.
What about just returning an integer and have a different way to get at 
TICKS-PER-SEC?
After all, the timer frequency is not specific to each measurement.

Ideally the returned value should be a fixnum to minimise overhead, but it may 
restrict the range on 32-bit platforms.

I also agree with Phillipp's comment about clock_gettime.