[gfortran,patch] Allow backtracing non-library errors

[FX Coudert]

> $ gfortran -g  -fbacktrace -ffpe-trap=invalid u3.f90 && ./a.out
>
> Program received signal 8 (SIGFPE): Floating-point exception.
>
> Backtrace for this error:
>   + /lib/i686/cmov/libc.so.6 [0xb7ce1208]
>   + /lib/i686/cmov/libc.so.6(__libc_start_main+0xe0) [0xb7ccd030]

If you remove -fbacktrace and catch the SIGFPE with gdb, what is the  
gdb backtrace?

FX