Upstream Bionic definitions of ctype_base/regex.

Summary:
This is a patch that Android has been carrying in its tree for several
years. This patch upstreams the existing ABI.

There's some historical cruft here. __regex_word used to be a part of
regex_traits rather than ctype_base. Bionic also used to use its own
ctype implementation because the libc++ builtin one wasn't available
yet. Bionic's ctype masks were 8 bits wide and already saturated, so a
wider type needed to be used for the regex mask, and the existing
value was already used so Android needed to specify its own.

Since then Android has migrated to the builtin ctype implementation
and this patch probably should have been dropped then. Unfortunately
that was not noticed at the time, so now we need to keep this to
maintain the current ABI.

Reviewers: EricWF, #libc, ldionne

Reviewed By: #libc, ldionne

Subscribers: dexonsmith, ldionne, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D76171