[Bug libstdc++/78870] Support std::filesystem on Windows

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78870

--- Comment #21 from Liu Hao <lh_mouse at 126 dot com> ---
On current branch master [1] and gcc-8-branch the libstdc++fs DLL is not
created for x86_64-w64-mingw32.

Please re-open.

[1]  remotes/origin/master                             84b3cf31c30  PR
target/85832     PR target/86036     * config/i386/sse.md
(<avx512>_eq<mode>3<mask_scalar_me