Use native-size integers for some global counters

Use native-size integers for some global counters

It may be unlikely that a Perl program will hit 2 billion SVs, but by
the time that 5.18 is ancient history, it's looking a lot more likely.
This makes two global counters use native-size ints.

I'm preserving signedness just for hysterical raisins: It might be
deliberate.