[POWERPC] Allow smp_call_function_single() to current cpu

This removes the requirement for callers to get_cpu() to check in simple
cases.  i386 and x86_64 already received a similar treatment.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>