Fix CUDA build after an optimistic patch in ff016691.
Shared OpenCL SHA256: Avoid a silly compiler warning on some devices/drivers.
Shared OpenCL SHA512: Work around some nearly unbelievable driver bugs.
I have yet to find a way to work around Catalyst 14.9 bugs for GCN. It does
a better job as a PRNG than as a GPGPU device. Maybe 14.12 "Omega" fixes
some bugs but frankly I would rather expect it to add more of them.