x86/shim: fix build when !PV32

x86/shim: fix build when !PV32

In this case compat headers don't get generated (and aren't needed).
The changes made by 527922008bce ("x86: slim down hypercall handling
when !PV32") also weren't quite sufficient for this case.

Try to limit #ifdef-ary by introducing two "fallback" #define-s.

Fixes: d23d792478db ("x86: avoid building COMPAT code when !HVM && !PV32")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>