vendor/golang_org/x/crypto/poly1305: sync to 1150b8bd09e53aea1d415621adae9bad665061a1

This change updates the vendored version of the poly1305 package to
match the latest version from x/crypto. This pulls in this change:

  commit 1150b8bd09e53aea1d415621adae9bad665061a1
  Author: Adam Langley <agl@golang.org>
  Date:   Fri Oct 21 15:59:10 2016 -0700

      poly1305: don't move R13 in sum_arm.s.

      Rather than change the value of R13 during the execution, keep R13 fixed
      (after the initial prelude) and always use offsets from it.

      This should help the runtime figure out what's going on if, say, a
      signal should occur while running this code.

      I've also trimmed the set of saved registers since Go doesn't require
      the callee to maintain anything except R10 and R13.

      Change-Id: Ifbeca73c1d964cc43bb7f8c20c61066f22fd562d
      Reviewed-on: https://go-review.googlesource.com/31717
      Run-TryBot: Cherry Zhang <cherryyz@google.com>
      TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Change-Id: I376b3e5d53aaded891e02801bd5faa5ff758da0d
Reviewed-on: https://go-review.googlesource.com/32227
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>