Update xxhash to r37.
Use XXH64 on 64-bit systems.

I'm not able to reproduce any real performance difference, but I know
there are some games which do more hashing and it may help there.
Merge pull request #7028 from unknownbrackets/xxhash-update

Update xxHash to r37, including XXH64
Merge https://github.com/hrydgard/ppsspp