x86: merge native_smp_prepare_cpus

With the previous changes, code for native_smp_prepare_cpus()
in i386 and x86_64 now look very similar. merge them into
smpboot.c. Minor differences are inside ifdef

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