x86: get rid of smp_boot_cpus

This patch get rid of smp_boot_cpus(), since it does not
boot any cpu anymore. Its code is split in a way to make
it closer to x86_64

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