[libcxx] [Windows] Pick a unique bit for __regex_word

The old `__regex_word` aliased the mask for `xdigit`, causing stray
test failures.

The diff may look surprising, as if the previous faulty value had
been set specifically for Windows - but this is due to a restructuring
in 411c630b. Prior to that, there
were OS specific settings for some OSes, and one fallback used for
the rest (which turns out to not work for Windows).

Differential Revision: https://reviews.llvm.org/D118188