x86: refactor ->enable_apic_mode() subarch methods

Only ES7000 has a real ->enable_apic_mode() method, the other
subarchitectures define it but keep it empty.

So mark the vector as NULL, extend the generic code to handle
NULL -setup_portio_remap() entries and remove all the empty
handlers.

Signed-off-by: Ingo Molnar <mingo@elte.hu>