x86/hypercall: fix gnttab hypercall args conditional build on pvshim

x86/hypercall: fix gnttab hypercall args conditional build on pvshim

A pvshim build doesn't require the grant table functionality built in,
but it does require knowing the number of arguments the hypercall has
so the hypercall parameter clobbering works properly.

Instead of also setting the argument count for the gnttab case if PV
shim functionality is enabled, just drop all of the conditionals from
hypercall_args_table, as a hypercall having a NULL handler won't get
to use that information anyway.

Note this hasn't been detected by osstest because the tools pvshim
build is done without debug enabled, so the hypercall parameter
clobbering doesn't happen.

Fixes: d2151152dd2 ('xen: make grant table support configurable')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>