[fortran, patch] Allow displaying backtraces from user code

[FX]

> I think that this approach is a mistake.  The patch
> starts us down a slippery slope.  Why not export all
> the nonstandard intrinsics?  In additions, the 
> _gfortran_ prefix was used to separate the libgfortran
> namespace from userspace.  Providing a means to 
> circumvent this separation seems to asking for more
> PR.

Well, the mean exists. All _gfortran_* functions can already be called, they're part of libgfortran's public (and versioned) API. I'm just saying adding a simple backtrace function to that list is useful, and documenting it too.

> I would rather see a new intrinsic procedure add to
> gfortran.  The standard does not prevent a vendor
> from offer additional intrinsic procedures not found
> in the standard.

I just think multiplicating vendor intrinsics makes our life harder. I'd like to hear other's opinions on this issue, but I'll implement the new intrinsic if that's the consensus.

Thanks,
FX