[libc++] Split re.alg tests into locale-dependent and independent tests

Currently all these tests are XFAILED on Linux even though the problem
only seems to be with the few checks that look at collation. To retain
test coverage this splits the locale-dependent tests into a separate
.pass.cpp that is XFAILed as before.
This commit also XFAILs the locale-dependent tests on FreeBSD since the
[=M=] and [.ch.] behaviour for cs_CZ also doesn't seem to match the
behaviour that is expected by these tests.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D94969