[patch,libgfortran,toplevel] Use libbacktrace in libgfortran

[FX]

> Awesome! Looks good. I only have one small bikeshed request: Can you
> make the output format match the existing code?

Problem with that is that ad the new backtrace uses full path for file names, the one-line format easily wraps in 80 column and make the output harder to read (in my opinion).

In realize this is mostly taste an very subjective, so with this nugget of extra input, I leave the final decision to you: let me know what you think best, between readability and "conformance" to GDB format.

Cheers,
FX