ON-16400: switch zf*pingpong to use rdtscp for more accurate latency measurement

[kieranm-xilinx]

I'd welcome some early review of the approach here: it looks like the zf_timer.h code is only used in zf[udp/tcp]pingpong, so I've made the change there, but happy to do it otherwise (e.g. in zf[udp/tcp]pingpong themselves) if there's a preference for that.

I'd also welcome suggestions for testing that reviewers would like to see before this gets merged, and possibly help with getting it done if it involves test infrastructure like jenkins jobs I don't have access to.

[kieranm-xilinx]

Some initial test results show a performance penalty from this change (which only affects our test apps, not customer code) of 9ns
Before: http://xcb-rb-logs.xilinx.com/users/kieranm/results/2025/09/09/0_dibench_10-39-51
After: http://xcb-rb-logs.xilinx.com/users/kieranm/results/2025/09/09/0_dibench_10-57-26

[jfeather-amd]

Looks good. Current testing seems sufficient to me, it would be good to see further testing as discussed elsewhere but happy that this can happen after this is merged.