cmd/dist: use standard generated code header

This belongs to a series of clean-up changes (see below) for cmd/dist.
This is change (4).

These changes include:
(1)  apply minor fixes
(2)  restore behavior of branchtag
(3)  unleash bootstrap optimization for windows
(4)  use standard generated code header
(5)  remove trivial variables + functions
(6)  move functions for the better
(7)  simplify code segments
(8)  use bytes.Buffer for code generation
(9)  rename variables + functions
(10) remove doc.go

Change-Id: I5e163f89a518f074e58bf2d44597e553c918d7e6
Reviewed-on: https://go-review.googlesource.com/61010
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/buildgo.go

@@ -26,7 +26,7 @@ import (
 // but we also write cmd/cgo/zdefaultcc.go
 func mkzdefaultcc(dir, file string) {
 	outGo := fmt.Sprintf(
-		"// auto generated by go tool dist\n"+
+		"// Code generated by go tool dist; DO NOT EDIT.\n"+
 			"\n"+
 			"package cfg\n"+
 			"\n"+
@@ -39,7 +39,7 @@ func mkzdefaultcc(dir, file string) {
 
 	// Convert file name to replace: turn go/internal/cfg into cgo.
 	outCgo := fmt.Sprintf(
-		"// auto generated by go tool dist\n"+
+		"// Code generated by go tool dist; DO NOT EDIT.\n"+
 			"\n"+
 			"package main\n"+
 			"\n"+
@@ -63,7 +63,7 @@ func mkzosarch(dir, file string) {
 	sort.Strings(list)
 
 	var buf bytes.Buffer
-	buf.WriteString("// auto generated by go tool dist\n\n")
+	buf.WriteString("// Code generated by go tool dist; DO NOT EDIT.\n\n")
 	buf.WriteString("package cfg\n\n")
 	fmt.Fprintf(&buf, "var OSArchSupportsCgo = map[string]bool{\n")
 	for _, plat := range list {
@@ -92,7 +92,7 @@ func mkzcgo(dir, file string) {
 	var buf bytes.Buffer
 
 	fmt.Fprintf(&buf,
-		"// auto generated by go tool dist\n"+
+		"// Code generated by go tool dist; DO NOT EDIT.\n"+
 			"\n"+
 			"package build\n"+
 			"\n"+

src/cmd/dist/buildruntime.go

@@ -24,7 +24,7 @@ import (
 //
 func mkzversion(dir, file string) {
 	out := fmt.Sprintf(
-		"// auto generated by go tool dist\n"+
+		"// Code generated by go tool dist; DO NOT EDIT.\n"+
 			"\n"+
 			"package sys\n"+
 			"\n"+
@@ -61,7 +61,7 @@ func mkzversion(dir, file string) {
 // original target (in this example, a Mac).
 func mkzbootstrap(file string) {
 	out := fmt.Sprintf(
-		"// auto generated by go tool dist\n"+
+		"// Code generated by go tool dist; DO NOT EDIT.\n"+
 			"\n"+
 			"package objabi\n"+
 			"\n"+

src/cmd/dist/buildtool.go

@@ -227,7 +227,9 @@ func bootstrapRewriteFile(text, srcFile string) string {
 	// binary that works for the current runtime.GOARCH.
 	// This saves 6+ seconds of bootstrap.
 	if archCaps, ok := isUnneededSSARewriteFile(srcFile); ok {
-		return fmt.Sprintf(`package ssa
+		return fmt.Sprintf(`// Code generated by go tool dist; DO NOT EDIT.
+
+package ssa
 
 func rewriteValue%s(v *Value) bool { panic("unused during bootstrap") }
 func rewriteBlock%s(b *Block) bool { panic("unused during bootstrap") }
@@ -262,7 +264,7 @@ func bootstrapFixImports(text, srcFile string) string {
 		}
 	}
 
-	lines[0] = "// Do not edit. Bootstrap copy of " + srcFile + "\n\n//line " + srcFile + ":1\n" + lines[0]
+	lines[0] = "// Code generated by go tool dist; DO NOT EDIT.\n// This is a bootstrap copy of " + srcFile + "\n\n//line " + srcFile + ":1\n" + lines[0]
 
 	return strings.Join(lines, "")
 }

src/cmd/dist/deps.go

-// generated by mkdeps.bash
+// Code generated by mkdeps.bash; DO NOT EDIT.
 
 package main
 

src/cmd/dist/mkdeps.bash

@@ -32,7 +32,7 @@ all="$(deps_of cmd/go | awk '{print $2}') cmd/go"
 deps_of $all >tmp.all.deps
 
 (
-	echo '// generated by mkdeps.bash'
+	echo '// Code generated by mkdeps.bash; DO NOT EDIT.'
 	echo
 	echo 'package main'
 	echo