crypto/sha512: Add AVX2 version for AMD64

name          old time/op    new time/op     delta
Hash8Bytes-6     913ns ± 0%      667ns ± 0%  -26.91%  (p=0.000 n=10+10)
Hash1K-6        6.58µs ± 0%     4.23µs ± 0%  -35.69%  (p=0.000 n=10+9)
Hash8K-6        45.9µs ± 0%     28.1µs ± 0%  -38.93%  (p=0.000 n=10+10)

name          old speed      new speed       delta
Hash8Bytes-6  8.76MB/s ± 0%  11.99MB/s ± 0%  +36.87%  (p=0.000 n=10+8)
Hash1K-6       156MB/s ± 0%    242MB/s ± 0%  +55.49%  (p=0.000 n=10+9)
Hash8K-6       178MB/s ± 0%    292MB/s ± 0%  +63.74%  (p=0.000 n=10+10)

Change-Id: Ic9211d68b02935b2195995f264ec57d6bc36f713
Reviewed-on: https://go-review.googlesource.com/36630
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/crypto/sha512/sha512block_amd64.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build amd64
+
+package sha512
+
+//go:noescape
+func blockAVX2(dig *digest, p []byte)
+
+//go:noescape
+func blockAMD64(dig *digest, p []byte)
+
+//go:noescape
+func checkAVX2() bool
+
+var hasAVX2 = checkAVX2()
+
+func block(dig *digest, p []byte) {
+	if hasAVX2 {
+		blockAVX2(dig, p)
+	} else {
+		blockAMD64(dig, p)
+	}
+}

src/crypto/sha512/sha512block_decl.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build amd64 s390x ppc64le
+// +build s390x ppc64le
 
 package sha512