[SPARC64]: Sun4v cross-call sending support.

Technically the hypervisor call supports sending in a list
of all cpus to get the cross-call, but I only pass in one
cpu at a time for now.

The multi-cpu support is there, just ifdef'd out so it's easy to
enable or delete it later.

Signed-off-by: David S. Miller <davem@davemloft.net>