x86: merge smp_32.h and smp_64.h into smp.h

Merge what's left from smp_32.h and smp_64.h into smp.h
By now, they're basically extern definitions.

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