[patch,libgfortran,toplevel] Use libbacktrace in libgfortran

[Uros Bizjak]

Hello!

> Use libbacktrace (instead of our own unwind-based code) to display backtraces from libgfortran
> upon error or user request.
>
>  1. In toplevel Makefile.def, make libgfortran depend on libbacktrace (needs global reviewer
> approval)
>   2. In gcc/fortran/config-lang.in, add libbacktrace to target_libs
>   3. In libgfortran, we remove our own code and substitute calls to libbacktrace
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu (which has full libbacktrace support) and
> x86_64-apple-darwin14 (which has minimal libbacktrace support). OK to commit to trunk?

backtrace.ChangeLog is unreadable for me ...

Uros.