x86: introduce smpboot_clear_io_apic

x86_64 has two nr_ioapics = 0 statements. In 32-bit, it can be done
too. We do it through the smpboot_clear_io_apic() inline function,
to cope with subarchitectures (visws) that does not compile mpparse in

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