Add a VIS-based block copy function for SPARC64 V and later, which

Add a VIS-based block copy function for SPARC64 V and later, which
additionally takes advantage of the prefetch cache of these CPUs.
Unlike the uncommitted US-III version, which provide no measurable
speedup or even resulted in a slight slowdown on certain CPUs models
compared to using the US-I version with these, the SPARC64 version
actually results in a slight improvement.