powerpc: Add VDSO version of getcpu

We have a request for a fast method of getting CPU and NUMA node IDs
from userspace. This patch implements a getcpu VDSO function,
similar to x86.

Ben suggested we use SPRG3 which is userspace readable. SPRG3 can be
modified by a KVM guest, so we save the SPRG3 value in the paca and
restore it when transitioning from the guest to the host.

I have a glibc patch that implements sched_getcpu on top of this.
Testing on a POWER7:

baseline: 538 cycles
vdso:      30 cycles

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

arch/powerpc/kernel/smp.c

@@ -48,6 +48,7 @@
 #ifdef CONFIG_PPC64
 #include <asm/paca.h>
 #endif
+#include <asm/vdso.h>
 #include <asm/debug.h>
 
 #ifdef DEBUG
@@ -570,6 +571,8 @@ void __devinit start_secondary(void *unused)
 #ifdef CONFIG_PPC64
 	if (system_state == SYSTEM_RUNNING)
 		vdso_data->processorCount++;
+
+	vdso_getcpu_init();
 #endif
 	notify_cpu_starting(cpu);
 	set_cpu_online(cpu, true);