Currently we emit errors whenever a map is not provided on a target data
directive, however, I believe that's incorrect behavior, the
specification states:
"At least one map, use_device_addr or use_device_ptr clause must appear
on the directive"
So provided one is present, the directive is legal in this case.
Slightly different to its siblings (enter/exit/update) which don't have
use_device_addr/use_device_ptr.
