test/codegen: add copyright headers to new codegen files

Change-Id: I9fe6572d1043ef9ee09c0925059ded554ad24c6b
Reviewed-on: https://go-review.googlesource.com/98215Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/codegen/bits.go

 // asmcheck
 
+// Copyright 2018 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.
+
 package codegen
 
 func bitcheck(a, b uint64) int {

test/codegen/rotate.go

 // asmcheck
 
+// Copyright 2018 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.
+
 package codegen
 
 import "math"