Update xxHash to r37, including XXH64

According to this benchmark, XXH64 is faster on 64-bit systems:
https://code.google.com/p/xxhash/

However, I wasn't able to reproduce any major difference using it (probably due to our amount of data being small.)  That being said, it wasn't slower, and presumably has some degree less collisions, and might be faster in certain games that using a lot of vertices.

This does however mean that vertex hashing may have different collisions on 32 vs 64 bit, but I don't think that's a big deal.

-[Unknown]