Add benchmark for the number of minimum cpu cores (#5127)

Fixes: https://github.com/paritytech/polkadot-sdk/issues/5122.

This PR extends the existing single core `benchmark_cpu` to also build a
score of the entire processor by spawning `EXPECTED_NUM_CORES(8)`
threads and averaging their throughput.

This is better than simply checking the number of cores, because also
covers multi-tenant environments where the OS sees a high number of
available CPUs, but because it has to share it with the rest of his
neighbours its total throughput does not satisfy the minimum
requirements.


## TODO
- [x] Obtain reference values on the reference hardware.

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Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>