[release-branch.go1.8] vendor/golang.org/x/crypto/curve25519: avoid loss of R15 in -dynlink mode

Original code fixed in https://go-review.googlesource.com/#/c/36359/.

Fixes #18820.

Change-Id: I060e6c9d0e312b4fd5d0674aff131055bf5cf61d
Reviewed-on: https://go-review.googlesource.com/36412


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-on: https://go-review.googlesource.com/36414


Reviewed-by: Austin Clements <austin@google.com>