cherry-picked from <https://github.com/Cyan4973/xxHash/pull/804>. Merge
commit is 449372ca8e0d1584d534dc82fe7d81ab75cfd6b4.
I cherry-picked that specific fix rather than trying to update vendored
xxhash to the latest version because there are quite a few
rocksdb-specific changes that have been made on top of upstream, so
pulling the newest version would be an involved process.