x86: provide specialized identification routines for x86_64

provide two specialized identify_secondary_cpu() and identify_boot_cpu()
routines for x86_64. Although not strictly needed, they are functionally
correct, and will ease integration with i386

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>