appengine was only necessary for the legacy system based on Go 1.9, drop
that. Add purego tags instead. Remove redundant architecture tags.
Change-Id: Ib1f65a4837511e63e08c1aa43163a79cfe868e0c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/215498
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>