[libcxx] [test] Fix the locale ctype widen tests on AIX (#91744)

The C locale on AIX uses `ISO-8859-1`, where `0xFB` is a valid
character. Widening char(-5) succeeds and produces L'\u00fb' the same as
on macOS, FreeBSD, and Windows. This patch removes `XFAIL:
LIBCXX-AIX-FIXME` and uses the macOS, FreeBSD, and WIN32 code path for
AIX.