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

[sjames at gcc dot gnu.org]

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

--- Comment #10 from Sam James <sjames at gcc dot gnu.org> ---
OK, avoiding speculation and getting back to facts:
```
$ cat ./build/x86_64-pc-linux-gnu/libstdc++-v3/src/c++23/relpath.log
called with '/usr/lib64' and
'/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits'
 -> called with '/usr/lib64' and
'/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits'
```

so toolexeclibdir is /usr/lib64 for me, and includebitsdir is
/usr/lib/gcc/x86_64-pc-linux-gnu/15/include/g++-v15/bits.