[XEN][POWERPC] SMP/IPI/MB combined

[XEN][POWERPC] SMP/IPI/MB combined
This patch rolls up and rebases the following patches for submission
against current tip of tree:
 * Memory barrier after SP store
 * IPI support
 * SMP support
The only changes from the previous submission other than trivial
fast-forward merges are to remove the ERAT flush, since it was comitted
seperately, and to make the status messages about waiting for remote
function completion ACKs only kick in after a full second has passed.
Note that this path REQUIRES that some form of the dom0 Linux patch
titled "Make Linux bail out of IPI vector reset" be applied.
Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>