cmd/internal/notsha256: add purego tag as needed

This permits building the package with gccgo, when using gccgo
as a bootstrap compiler.

Fixes #53662

Change-Id: Ic7ae9323ec5954e9306a32e1160e9aa1ed3aa202
Reviewed-on: https://go-review.googlesource.com/c/go/+/415935
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/notsha256/sha256block_386.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !purego
+// +build !purego
+
 // SHA256 block routine. See sha256block.go for Go equivalent.
 //
 // The algorithm is detailed in FIPS 180-4:

src/cmd/internal/notsha256/sha256block_amd64.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !purego
+// +build !purego
+
 package notsha256
 
 var useAVX2 = false

src/cmd/internal/notsha256/sha256block_amd64.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !purego
+// +build !purego
+
 #include "textflag.h"
 
 // SHA256 block routine. See sha256block.go for Go equivalent.

src/cmd/internal/notsha256/sha256block_decl.go

@@ -2,7 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build 386 || amd64 || ppc64le || ppc64
+//go:build !purego && (386 || amd64 || ppc64le || ppc64)
+// +build !purego
 // +build 386 amd64 ppc64le ppc64
 
 package notsha256

src/cmd/internal/notsha256/sha256block_generic.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !amd64 && !386 && !ppc64le && !ppc64
-// +build !amd64,!386,!ppc64le,!ppc64
+//go:build purego || (!amd64 && !386 && !ppc64le && !ppc64)
+// +build purego !amd64,!386,!ppc64le,!ppc64
 
 package notsha256
 

src/cmd/internal/notsha256/sha256block_ppc64x.s

@@ -8,7 +8,8 @@
 // bootstrap toolchain.
 //
 
-//go:build ppc64 || ppc64le
+//go:build !purego && (ppc64 || ppc64le)
+// +build !purego
 // +build ppc64 ppc64le
 
 // Based on CRYPTOGAMS code with the following comment: