This change updates the vendored copy of x/crypto/curve25519,
specifically to include the following changes:
  f620851 curve25519: eliminate unnecessary "callee save" prologues
  722a7b7 curve25519: fix confusing SP adjustments

Change-Id: I1b16aba12c744ac32f925654a136a8f52cd40fc2
Reviewed-on: https://go-review.googlesource.com/31666
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>