This change updates the vendored copy of x/crypto/poly1305, specifically
to include the following changes:
  3ded668 poly1305: enable assembly for ARM in Go 1.6.
  dec8741 poly1305: fix stack handling in sum_arm.s

Fixes #17499.

Change-Id: I8f152da9599bd15bb976f630b0ef602be05143d3
Reviewed-on: https://go-review.googlesource.com/31592
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>