[perl #116907] Allow //g matching past 2**31 threshold

[perl #116907] Allow //g matching past 2**31 threshold

Change the internal fields for storing positions so that //g in scalar
context can move past the 2**31 character threshold.  Before this com-
mit, the numbers would wrap, resulting in assertion failures.

The changes in this commit are only enough to get the added test pass-
ing.  Stay tuned for more.