xoroshiro RNG doesn't match reference

Our xoroshiro128+ generator has a link to the C source code. However, if you follow that link, the actual C code doesn't match the C code we pasted in as a comment.

Not sure what happened, but I suspect we were using an early version that got updated at some point. Modifying it shouldn't be too hard.

But if we're modifying the RNG (again), perhaps we should update to xoroshiro256++. I don't think it would be terribly hard to implement this. There are 256 bits of state vs the 128 bits we currently have.

Of course, we have an issue that we don't have a current sparc box to test with. Whatever we do will be for x86 only. At least for now.