"bench native" should be "bench native=on"

[SkySkimmer]

As discussed in https://github.com/coq/coq/wiki/Coq-Call-2022-11-02

[ejgallego]

Even maybe we could call that native_cmxs_gen=on?

But indeed, the problem of the number being quite meaningless in terms of performance still exists, IMHO we need to rethink the bench setup for this to be any useful.