[regex] Use distinct __regex_word on NetBSD

NetBSD defines character classes up to 0x2000.  Use 0x8000 as a safe
__regex_word that hopefully will not collide with other values
in the foreseeable future.

Differential Revision: https://reviews.llvm.org/D55657

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349293 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed