[Bug libstdc++/119266] libstdc++.modules.json has wrong path

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119266

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes for the overwhelming majority of users, it would make sense for
contrib/relpath.sh (or libstdc++-v3/src/c++23/Makefile.am) to use GNU realpath
when available.

https://www.gnu.org/software/coreutils/manual/html_node/Realpath-usage-examples.html

realpath --version 2>/dev/null | grep 'realpath (GNU coreutils)'

It looks like the --relative-to option was present from the first version of
GNU realpath, released in coreutils v8.15 in 2012.

But relpath.sh probably needs to be fixed for targets without GNU coreutils.