tools/ocaml: Fix redefinition errors

tools/ocaml: Fix redefinition errors

If PAGE_SIZE is already defined in the system (e.g. in /usr/include/limits.h
header) then gcc will trigger a redefinition error because of -Werror. This
patch replaces usage of PAGE_* macros with XC_PAGE_* macros in order to avoid
confusion between control domain page granularity (PAGE_* definitions) and
guest domain page granularity (which is what we are dealing with here).

Same issue applies for redefinitions of Val_none and Some_val macros which
can be already define in the OCaml system headers (e.g.
/usr/lib/ocaml/caml/mlvalues.h).

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Ian Jackson <iwj@xenproject.org>
Tested-by: Dario Faggioli <dfaggioli@suse.com>