x86: apic - introduce get_physical_broadcast for 64bit

We don't really use it now on 64bit mode but
could reserve it for future.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>