[Bug libstdc++/81751] __basic_file::sync() may flush _all_ files

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81751

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Or just call fflush(__file) directly:

https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00673.html