make do_sched_op_compat() x86-specific

make do_sched_op_compat() x86-specific

Being a pre-3.1 compatibility hypercall handler only, it's not needed
on ARM or any future architectures Xen may get ported to.

Also the function shouldn't really be used internally - its use should
be limited to its purpose (and hence there's also no need for a
prototype).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@cirix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>