[Bug libstdc++/91947] std::filesystem::file_size will return wrong value on 32bit platforms with large files support

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91947

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's done the same way as we've always created the ${host_builddir}/c++config.h
target, so I'm not sure why one is racy and the other isn't.