xen: move -nostdinc into common Rules.mk.

xen: move -nostdinc into common Rules.mk.

Previously we didn't use it at all the on ARM ports or for clang builds.

For ARM, I think this is just an oversight.

For clang, this used not to work, because '-withprefix include' didn't
let us see stdarg.h, but that's fixed in clang v3.0.

Also move the '-withprefix include' to beside -nostdinc as it's only
needed with -nostdinc anyway.

Signed-off-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>