powerpc: Select either privileged or hypervisor doorbell when sending

On book3s we have two msgsnd instructions with differing privilege
levels. This patch selects the appropriate instruction to use whenever
we send a doorbell interrupt.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Tested-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>