Remove endian-sensitivity from hash slow path

Remove endian-sensitivity from hash slow path

Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/9c6a50fdd80bb39fabd95faeda84f04062685ff3

Prior to this commit, the Abseil hash fast path was endian-agnostic, but
the slow path assumed a little-endian platform. Change the slow path to
be endian-correct, ensuring that values produced by the fast and slow
paths are equal even on big-endian systems.

The author works at Google. Upstream applied this patch as Piper revision
355424258 and exported it to GitHub; the Applied-Upstream URL above points to
the exported commit.

Gbp-Pq: Name endian-hash.diff