x86: use x2apic id reported by cpuid during topology discovery

use x2apic id reported by cpuid during topology discovery, instead of the
apic id configured in the APIC. For most of the systems, x2apic id
reported by cpuid leaf 0xb will be same as the physical apic id reported
by the APIC_ID register of the APIC. We follow the suggested guidelines
and use the apic id reported by the cpuid.

No change to non-generic UV platforms, will use the apic id reported in the
APIC_ID register as the cpuid reported apic id's may not be unique.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>