cpu: Make kvm-stub.o available outside softmmu

It will provide stubs for *-user targets once softmmu-specific calls
are attempted from common CPU code.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>