cmd/go: fix bytes and net the right way

Not sure why they used empty.s and all these other
packages were special cased in cmd/go instead.
Add them to the list.

This avoids problems with net .s files being compiled
with gcc in cgo mode and gcc not supporting // comments
on ARM.

Not a problem with bytes, but be consistent.

The last change fixed the ARM build but broke the Windows build.
Maybe *this* will make everyone happy. Sigh.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/144530046

src/bytes/bytes.s

-// 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.
-
-// This file is here just to make the go tool happy.

src/cmd/go/build.go

@@ -1630,7 +1630,7 @@ func (gcToolchain) gc(b *builder, p *Package, archive, obj string, importArgs []
 	extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.CXXFiles) + len(p.MFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
 	if p.Standard {
 		switch p.ImportPath {
-		case "os", "runtime/pprof", "sync", "time":
+		case "bytes", "net", "os", "runtime/pprof", "sync", "time":
 			extFiles++
 		}
 	}

src/net/empty.s

-// 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 !cgo
-
-// This file is required to prevent compiler errors
-// when the package built with CGO_ENABLED=0.
-// Otherwise the compiler says:
-// pkg/net/fd_poll_runtime.go:15: missing function body