xen: build infrastructure for generating hypercall depending symbols

Today there are several places in the kernel which build tables
containing one entry for each possible Xen hypercall. Create an
infrastructure to be able to generate these tables at build time.

Based-on-patch-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>