[Bug libstdc++/86447] gcc 9.0 from r262456 can't build cross compiler for mingw-w64 target

[mateuszb at poczta dot onet.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86447

--- Comment #1 from mateuszb at poczta dot onet.pl ---
This bug is when you configure GCC with option
--enable-fully-dynamic-string