x86, smp: refactor ->inquire_remote_apic() methods

Nothing exciting - a few subarches dont want APIC remote reads to
be performed - the others are content with the default method.

 - extend the generic code to handle NULL methods

 - clear out dummy methods and replace them with NULL

 - clean up: remove wrapper macros, etc.

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