[patch,libgfortran,toplevel] Use libbacktrace in libgfortran

[FX]

>> 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 …

Sending again, this time with .txt extension, hoping this makes it go through OK.

FX

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