[Bug libstdc++/64571] [5.0 Regression] Link failure with fstream due to new string implementation

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64571

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
They are defined in src/c++11/fstream-inst.cc so I think I just need to add the
new symbols to the linker script. I'm surprised the testsuite didn't catch it
though!