generalize the x2apic code some more.

let read_apic_id become a macro (later on a function/inline)
GET_APIC_ID(apic_read(APIC_ID))

  +#define read_apic_id()  (GET_APIC_ID(apic_read(APIC_ID)))

instead of this weird construct:

  -#define read_apic_id  (genapic->read_apic_id)
Signed-off-by: NYinghai Lu <yhlu.kernel@gmail.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>