regex: don't assume uint64_t or uint32_t

This avoids -Werror=overflow errors for 32-bit systems in
the 64-bit case.  Problem reported by Joseph Myers in:
https://sourceware.org/ml/libc-alpha/2017-11/msg00694.html
Also, when this code is used in Gnulib it ports to platforms
that lack uint64_t and uint32_t.  The C standard doesn't guarantee
them, and on some 32-bit compilers there is no uint64_t.
Problem reported by Gianluigi Tiesi in:
http://lists.gnu.org/archive/html/bug-gnulib/2012-03/msg00154.html
* posix/regcomp.c (init_word_char): Don't assume that the types
uint64_t and uint32_t exist.  Adapted from Gnulib patch
2012-05-27T06:40:00!eggert@cs.ucla.edu.  See:
https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=252b52457da7887667c036d18cc5169777615bb0