argon2: add missing amd64 tags

The change fixes argon2_test build failure which currently
reads as follows when built with gccgo:

./argon2/blamka_ref.go:9:1: error: redefinition of 'processBlock'
 func processBlock(out, in1, in2 *block) {
 ^
./argon2/blamka_amd64.go:53:1: note: previous definition of 'processBlock' was here
 func processBlock(out, in1, in2 *block) {
 ^
./argon2/blamka_ref.go:13:1: error: redefinition of 'processBlockXOR'
 func processBlockXOR(out, in1, in2 *block) {
 ^
./argon2/blamka_amd64.go:57:1: note: previous definition of 'processBlockXOR' was here
 func processBlockXOR(out, in1, in2 *block) {
 ^

Change-Id: I5c1d7360a2a91a2ac27273a5c1da2a139b1537a7
Reviewed-on: https://go-review.googlesource.com/88455
Run-TryBot: Alex Vaghin <ddos@google.com>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

argon2/blamka_amd64.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build amd64,!gccgo,!appengine
+
 package argon2
 
 func init() {