curve25519: avoid loss of R15 in -dynlink mode

Fixes golang/go#18820.

Change-Id: I4b3a49b3bbbecc4e1008989fefd39da9725a28ea
Reviewed-on: https://go-review.googlesource.com/36359
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Adam Langley <agl@golang.org>
6 files changed