Make regex_word not clash with another character type.

This makes the re.traits/isctype.pass.cpp test pass.

regex_traits::__regex_word was defined to 0x80 which was not supposed to be
covered by existing ctype macros (e.g. _CTYPE_S, _CTYPE_R...).

However Android's whitespace bit, aka. _B (see locale_android.cpp for the
details) was unfortunately defined as 0x80 which made the whitespace character
be recognized as a word.

Change-Id: I3142484e7f1202700bc3d95081e402e32ea07f2a
Add a comment above __regex_word to make upstreaming easier.

Change-Id: I5aefc86c8d79077c827e247da2b74bbf4e96c200
Merge pull request #87 from awong-dev/pliard-change-value-of-regex-word

Make regex_word not clash with another character type.