'num_gangs', 'num_workers', and 'vector_length' are similar to
eachother, and are all the same implementation as for compute
constructs, so this patch just enables them and adds the necessary
testing to ensure they work correctly. These will get more complicated
when they get combined with 'gang', 'worker', 'vector' and 'reduction',
but those restrictions will be implemented when those clauses are
enabled.
