x86: provide an end_local_APIC_setup function

It splits setup_local_APIC in two, providing a function corresponding
to the ending part of it. As a side effect, smp_callin looks the same
between i386 and x86_64.

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