all: add //go:build lines to assembly files

For golang/go#41184

Change-Id: Ica67fdbf2745ad2eef63dbb9ef70136e9e6fd348
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/319469
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

blake2b/blake2bAVX2_amd64.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.7 && amd64 && gc && !purego
 // +build go1.7,amd64,gc,!purego
 
 #include "textflag.h"

blake2b/blake2b_amd64.s

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build amd64 && gc && !purego
 // +build amd64,gc,!purego
 
 #include "textflag.h"