xen/compat: support XEN_HAVE_FOO ifdefs in public interface

xen/compat: support XEN_HAVE_FOO ifdefs in public interface

This allows us expose or hide interface features on different architectures
without requiring nasty arch-specific ifdeffery.

Preserves any #ifdef with a XEN_HAVE_* symbol name, as well as any #else or

The ifdef symbol becomes COMPAT_HAVE in the compat versions so that
architectures can enable or disable interfaces for compat mode too. (This
actually just fell out of the way the existing stuff works and it didn't seem
worth jumping through hoops to make the name remain XEN_HAVE).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>