Update xxhash implementation from https://github.com/Cyan4973/xxHash.
This has moved a lot of code so the diff is huge, plus the code we don't
need now for btrfs has been removed (XXH3, XXH32).
There's no significant change in performance.
Signed-off-by: David Sterba <dsterba@suse.com>
Copy of xxhash.[ch] from git://github.com/Cyan4973/xxHash, version
v0.7.1. The include xxh3.h has been commented out as we don't have it
here, otherwise the copy is unchnaged.
Signed-off-by: David Sterba <dsterba@suse.com>
