checkasm: Add an --affinity= option for selecting a CPU core

Add an option for selecting the core where the single thread of checkasm runs. This allows benchmarking on specific CPU cores on heterogenous CPUs, like ARM big.LITTLE configurations.

On Linux, one can easily wrap an invocation of checkasm with "taskset -c [...]" - so this option isn't very essential there - however it is quite useful on Windows.

On Windows, it is somewhat possible to do the same by launching the tool with "start /B /affinity [...]", but that doesn't work well with scripting ("start" returns before the command has finished running, and it's not obvious how to invoke "start" from within WSL).

Using "taskset" to launch processes on specific cores within WSL on Windows doesn't work - regardless of the Linux level affinity, the process ends up running on the performance cores anyway.