hw/cpu: core.c can be compiled as common object

There does not seem to be any target specific code in core.c, so we can
put it into "common-obj" instead of "obj" to compile it only once for
all targets.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>