Export cpu topology info via sysfs. Items (attributes) are similar

  to /proc/cpuinfo.
  
  1) /sys/devices/system/cpu/cpuX/topology/physical_package_id:
  represent the physical package id of  cpu X;
  2) /sys/devices/system/cpu/cpuX/topology/core_id:
  represent the cpu core id to cpu X;
  3) /sys/devices/system/cpu/cpuX/topology/thread_siblings:
  represent the thread siblings to cpu X in the same core;
  4) /sys/devices/system/cpu/cpuX/topology/core_siblings:
  represent the thread siblings to cpu X in the same physical package;
  
  To implement it in an architecture-neutral way, a new source file,

  drivers/base/topology.c, is to export the 4 attributes.

  For an architecture to support this feature, it must define some of
  these macros in include/asm-XXX/topology.h:

  #define topology_physical_package_id(cpu)
  #define topology_core_id(cpu)
  #define topology_thread_siblings(cpu)
  #define topology_core_siblings(cpu)
  
  The type of **_id is int.
  The type of siblings is cpumask_t.

  To be consistent on all architectures, include/linux/topology.h
  provides default definitions for any of the above macros that are
  not defined by include/asm-XXX/topology.h:
  1) physical_package_id: -1
  2) core_id: 0
  3) thread_siblings: just the given CPU
  4) core_siblings: just the given CPU