implement a caml_get_current_callstack primitive to save the call stack at any point
    
    This commit only implements runtime support in asmrun/ and byterun/.
    The more subjective Printexc interface will come separately.
    
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@13819 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02