[libc++] mdspan - implement layout_stride (#69650)

This implements layout_stride for C++23 and with that completes the
implementation of the C++23 mdspan header. The feature test macro is
added, and the status pages updated.

Co-authored-by: Damien L-G <dalg24@gmail.com>

Differential Revision: https://reviews.llvm.org/D157171