blake2b: fix build on non-amd64 platforms

Change-Id: Ib9ebb1a2eff4b61f60453086be5c63ac7af1f7fc
Reviewed-on: https://go-review.googlesource.com/34672
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@golang.org>
6 files changed