[fortran, patch] Allow displaying backtraces from user code

PR 36044 (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36044) is an enhancement request for a way to display backtraces from user code. I'm against adding yet another nonstandard intrinsic for this purpose (which is how Intel Fortran does it), but I would like to offer the following solution to the issue, as I think it can be useful in some cases (and the way I suggest should not be a maintainance burden for us):

  -- export _gfortran_show_backtrace() from libgfortran (instead of it being an internal function)
  -- add documentation on how to call this function from user-code using BIND(C)

Patch was bootstrapped and regtested on x86_64-apple-darwin11, also tested with "make info html pdf". OK for trunk?
FX