powerpc: Reduce footprint of xics_ipi_struct

powerpc: Reduce footprint of xics_ipi_struct

Right now we allocate a cacheline sized NR_CPUS array for xics IPI
communication. Use DECLARE_PER_CPU_SHARED_ALIGNED to put it in percpu
data in its own cacheline since it is written to by other cpus.

On a kernel with NR_CPUS=1024, this saves quite a lot of memory:

   text    data     bss      dec         hex    filename
8767779 2944260 1505724 13217763         c9afe3 vmlinux.irq_cpustat
8767555 2813444 1505724 13086723         c7b003 vmlinux.xics

A saving of around 128kB.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>