x86: slim down hypercall handling when !PV32

x86: slim down hypercall handling when !PV32

In such a build various of the compat handlers aren't needed. Don't
reference them from the hypercall table, and compile out those which
aren't needed for HVM. Also compile out switch_compat(), which has no
purpose in such a build.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>