all: add go:build lines (#23468)

Generated by go1.17 fmt ./...

crypto/bn256/bn256_fast.go

@@ -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 || arm64
 // +build amd64 arm64
 
 // Package bn256 implements the Optimal Ate pairing over a 256-bit Barreto-Naehrig curve.

crypto/bn256/bn256_slow.go

@@ -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 && !arm64
 // +build !amd64,!arm64
 
 // Package bn256 implements the Optimal Ate pairing over a 256-bit Barreto-Naehrig curve.

crypto/bn256/cloudflare/gfp_decl.go

@@ -1,3 +1,4 @@
+//go:build (amd64 && !generic) || (arm64 && !generic)
 // +build amd64,!generic arm64,!generic
 
 package bn256

crypto/bn256/cloudflare/gfp_generic.go

@@ -1,3 +1,4 @@
+//go:build (!amd64 && !arm64) || generic
 // +build !amd64,!arm64 generic
 
 package bn256