[gfortran,patch] Allow backtracing non-library errors

[FX Coudert]

:ADDPATCH fortran:

> the patch compiles and regtests ok on i686-pc-linux-gnu (Debian
> testing).
>
> I can't get it to produce meaningful backtraces, though

The answer I got from the glibc mailing-list is that it's a bug in  
glibc or/and the kernel, which is likely to have been fixed in later  
glibc/kernel releases. What's more, the backtrace patch gives  
meaningful backtraces except for the top frame; for our small  
testcases, it's quite unusable, but for more complex programs, where  
the backtrace can contain hundreds of frames, it's probably of less  
importance. With these reasons in mind, I'd like to ask for a review  
of that patch, in its current state. Of course, if there emerge a  
consensus that it's not the right thing to do, I'll withdraw it.

PS: Special offer for the second week-end of august: with all the  
people on vacation, consensus starts at 1 maintainer! ;-)

Bootstrapped and regtested on x86_64-linux, OK for mainline?

FX