[arm] Implement the byte swap operations (PR#5774).

Use rev16/movt to implement direct 16-bit byte-swap operation
(ARMv6T2 and above) and rev to implement the 32-bit byte-swap
operation (ARMv6 and above).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13203 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02