[POWERPC] Fix corruption in hcall9

It looks to me like we are corrupting r12 in the hcall9 function.
Although we have r0 free we cant use offsets against it, so save
away r12 in there instead.  r12 holds the ninth return value from
the hypervisor call, so without this fix, the caller will see the
wrong value for the ninth element in the array that gets the return
values.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>