Support nvcc's --default-stream option

[trxcllnt]

From nvcc --help:

--default-stream {legacy|null|per-thread}       (-default-stream)               
        Specify the stream that CUDA commands from the compiled program will be sent
        to by default.
                
        legacy
                The CUDA legacy stream (per context, implicitly synchronizes with
                other streams).
                
        per-thread
                A normal CUDA stream (per thread, does not implicitly
                synchronize with other streams).
                
        'null' is a deprecated alias for 'legacy'.
                
        Allowed values for this option:  'legacy','null','per-thread'.
        Default value:  'legacy'.

cc @robertmaynard