Reliable time measuring. I need a very cache hostile function, that should not be too short either. A normal `nop` might do nothing (not even fill up a space in the pipeline). I cannot use branches or access the memory because that might create unreliable timings. [nop_nop_nop](https://github.com/Kijewski/RIOT/blob/implement-x86/cpu/x86/nop_nop_nop.inc) just uses simple arithmetic that cannot be optimized by the CPU at runtime.