Update "pg_regress --no-locale" for Darwin and Windows.

Commit 894459e5 revealed this option to
be broken for NLS builds on Darwin, but "make -C contrib/unaccent check"
and the buildfarm client rely on it.  Fix that configuration by
redefining the option to imply LANG=C on Darwin.  In passing, use LANG=C
instead of LANG=en on Windows; since only postmaster startup uses that
value, testers are unlikely to notice the change.  Back-patch to 9.0,
like the predecessor commit.