reinstate the deprecated primitive caml_get_exception_backtrace
    
    Jacques-Henri initially removed the primitive, which is deprecated
    since 4.01, but I suspect there still are uses in the wild. I guess we
    should wait for a few more versions.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14780 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02