x86: change GET_APIC_ID() from an inline function to an out-of-line function

Introduce a function to read the local APIC_ID.

This change is in preparation for additional changes to
the APICID functions that will come in a later patch.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>