[gnuregex] Fix size demotion rounding errors that have crept up with later gcc

[adrian]

Summary

This fixes some fun type size truncation that shows up giving errors like
" changes value from '287948901175001088' to '0' "

.. which turns out is a known issue with later gcc's.

eg - https://lists.gnu.org/archive/html/bug-gnulib/2012-03/msg00154.html

It was fixed up upstream corelib/gnulib in commit hahs
252b52457da7887667c036d18cc5169777615bb0
(eg in https://github.com/coreutils/gnulib/commit/252b52457da7887667c036d18cc5169777615bb0)

Test Plan

• compiled/run for gcc-6.4 on amd64

[adrian]

1. Updating D26804: [gnuregex] Fix size demotion rounding errors that have crept up with later gcc #
2. Enter a brief description of the changes included in this update.
3. The first line is used as subject, next lines as comment.

whoops, didn't include the diff

[imp]

seems legit