Update the poly1305 and curve25519 packages to the current state of /x/crypto.

Updates #19967

Change-Id: Ib71534f78040f31bfd5debb06f3c4a54a77955b3
Reviewed-on: https://go-review.googlesource.com/40711Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>