[libcxx][test] Fix numpunct grouping tests on AIX (#91781)

The `grouping` string for locale `en_US.UTF-8` and `fr_FR.UTF-8` on AIX
is `3`. This is different from Linux's `3;3` but is the same as Windows.
This patch removes `XFAIL: LIBCXX-AIX-FIXME` and changes to use the
`WIN32` code path.