cmd/dist: apply minor fixes

This belongs to a series of clean-up changes (see below) for cmd/dist.
This is change (1).

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: I49e5f2a9b6146e2b60a067da5bac31434ffc9aaf
Reviewed-on: https://go-review.googlesource.com/60650Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/buildgo.go

@@ -22,7 +22,7 @@ import (
 //	const defaultCXX = <defaultcxx>
 //	const defaultPkgConfig = <defaultpkgconfig>
 //
-// It is invoked to write cmd/go/zdefaultcc.go
+// It is invoked to write cmd/go/internal/cfg/zdefaultcc.go
 // but we also write cmd/cgo/zdefaultcc.go
 func mkzdefaultcc(dir, file string) {
 	outGo := fmt.Sprintf(
@@ -101,7 +101,7 @@ func mkzcgo(dir, file string) {
 	for _, plat := range list {
 		fmt.Fprintf(&buf, "\t%q: true,\n", plat)
 	}
-	fmt.Fprintf(&buf, "}")
+	fmt.Fprintf(&buf, "}\n")
 
 	writefile(buf.String(), file, writeSkipSame)
 }