implement a caml_stash_current_backtrace 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/trunk@13813 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
bootstrap after the new primitives


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13814 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02