x86: take load_sp0 out of smpboot.c

there's no particular reason to do load_sp0 in different
places for i386 and x86_64. They should all be in cpu_init.
Right now, cpu_init itself is not integrated, but with this patch,
the code becomes closer to each other, making in easier to integrate
when the time comes.

Furthermore, although doing it in do_boot_cpu for x86_64 is fine, since it's
only a copy, load_sp0 should be executed in the cpu it refers to anyway.

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