x86: surround hard_smp_processor_id in APIC_DEFINITION

APIC_DEFINITION is not defined in x86_64, so in practice, we keep
our old code here. But as a nice side effect, the code is now
equal to smp_32.h.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>