Add a different sort of stats_num_field helper function

Uses look like
    stats_num_field('bytes allocated',
                    [(wordsize(32), 45648, 5),
                     (wordsize(64), 49400, 5)])
where the first matching triple will be used. e.g. we could override
the Win32 expected values with:
                    [(platform('i386-unknown-mingw32'), 41000, 5),
                     (wordsize(32),                     45648, 5),
                     (wordsize(64),                     49400, 5)])
with other 32-bit platforms falling through to the wordsize(32) case.

This makes it easier to give different values for different platforms,
while being sure that all platforms are covered.
Allow a simpler form of stats_num_field where all platforms use the same value
Convert to stats_num_field in perf/should_run