[libc++] Properly define _LIBCPP_HAS_NO_UNICODE in __config_site (#95138)

Fixes #93638

Co-authored-by: Mark de Wever <koraq@xs4all.nl>
NOKEYCHECK=True
GitOrigin-RevId: 04a75f54a11f1f2640a211a81120966c7a24d05b
14 files changed