hw/core: Move null-machine into the common-obj list

hw/core: Move null-machine into the common-obj list

The null-machine code used to be target specific since it used the
target-specific cpu_init() function in the past. But in the recent
commit 2278b93941d42c30e2950 ("Use cpu_create(type) instead of
cpu_init(cpu_model)") this has been change, so that the code now
uses the common cpu_create() function instead. Thus we can put
the null-machine into the common-obj list so that it is compiled
only once for all targets, to save some compilation time.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>