metag: topology: export 'cpu_core_map'

We need to export 'cpu_core_map' since the topology_core_cpumask macro
refers to it and is used by certain kernel modules.

Found in allmodconfig build:
  ERROR: "cpu_core_map" [drivers/staging/lustre/lustre/libcfs/libcfs.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: James Hogan <james.hogan@imgtec.com>