Port to the ARM 64-bits (AArch64) architecture (experimental). Merge of branch branches/arm64.
git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13909 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
(cherry picked from commit 055d5c0379e42b4f561cb1fc5159659d8e9a7b6f)