Move the benchmarking logic out into a script.

Move the benchmarking logic out into a script.

It's a pain having it in a rule in Makefile.gtk, which isn't even the
recommended makefile these days - it can't be re-run conveniently, and
there's no way to parametrise it. Now it can be run no matter which
makefile you're using, and it lets you narrow down to a subset of
games (though not presets). Other options could easily be added.