add getCpuTicks based on RDTSC instruction for highest-performance counters

[timotheecour]

links

• would solve randomPathName called twice in a row can return the same string on windows nim-lang/Nim#17898 without having to resort to {.threadvar.} or atomics
• would improve resolution of CPU profiler (eg nimprof)
• would improve accuracy of benchmarks, without needing to run through lots of loops
• would avoid problems due to CPU speed variations (eg under load) to a large extent
• would replace advantageously cpuTime for most benchmarking applications
• D20210609T004849

note

I have a PR in the works