x86: change x86_64 native_smp_prepare_cpus to match i386

An APIC test is moved, and code is replaced by the mach-default
already defined function (smpboot_setup_io_apic).
setup_portio_remap() is added, but it is a nop in mach-default.

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