[Bug libstdc++/85768] [9 Regression] FreeBSD bootstrap fails due to undefined reference to 'backtrace'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85768

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes I did suggest that. For now I think we should just disable the use of
backtrace anywhere except GNU/Linux.