asm-generic: add __smp_xxx wrappers

On !SMP, most architectures define their
barriers as compiler barriers.
On SMP, most need an actual barrier.

Make it possible to remove the code duplication for
!SMP by defining low-level __smp_xxx barriers
which do not depend on the value of SMP, then
use them from asm-generic conditionally.

Besides reducing code duplication, these low level APIs will also be
useful for virtualization, where a barrier is sometimes needed even if
!SMP since we might be talking to another kernel on the same SMP system.

Both virtio and Xen drivers will benefit.

The smp_xxx variants should use __smp_XXX ones or barrier() depending on
SMP, identically for all architectures.

We keep ifndef guards around them for now - once/if all
architectures are converted to use the generic
code, we'll be able to remove these.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>