Impact: build fix

wakeup_secondary_cpu_via_init(), the default platform method for
booting a secondary CPU, is always used on UP due to probe_32.c,
if CONFIG_X86_LOCAL_APIC is enabled but SMP is off.

So provide a UP wrapper inline as well.

Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>