OpenCL shared SHA256: Optimize memcpy for 30-50% boost of 7z-opencl,
plus some device-specific tweaks. An alternate memcpy is defined out
right now but it may be better IRL (or once adding some test vectors
of varying length to 7z).

Shared OpenCL SHA2: Alternate memcpy. This seems slower but might be faster
with more varying test vectors so keeping for now, but defined out.