x86: AMD core-pair topology detection code

This patch is to support core-pair topology introduced by AMD CPUs, 
which introduces a new concept of [core, compute unit]. There is a new 
feature bit for topology extension in CPUID:0x80000001. Also a new CPUID 
0x8000001E is introduced for CPU topology enumeration. This patch 
collects the sibling information from the new CPUID and will be stored 
in the sibling map in Xen hypervisor.

Signed-off-by: Wei Huang <wei.huang2@amd.com>