cmd/dist: sort entries in zcgo.go generated file for deterministic build

This simplifies comparison of object files across different builds
by ensuring that the strings in the zcgo.go always appear in the
same order.

Change-Id: I3639ea4fd10e0d645b838d1bbb03cd33deca340e
Reviewed-on: https://go-review.googlesource.com/22478Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/dist/buildgo.go

@@ -7,6 +7,7 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"sort"
 )
 
 /*
@@ -48,6 +49,15 @@ func mkzdefaultcc(dir, file string) {
 //
 // It is invoked to write go/build/zcgo.go.
 func mkzcgo(dir, file string) {
+	// sort for deterministic zcgo.go file
+	var list []string
+	for plat, hasCgo := range cgoEnabled {
+		if hasCgo {
+			list = append(list, plat)
+		}
+	}
+	sort.Strings(list)
+
 	var buf bytes.Buffer
 
 	fmt.Fprintf(&buf,
@@ -56,10 +66,8 @@ func mkzcgo(dir, file string) {
 			"package build\n"+
 			"\n"+
 			"var cgoEnabled = map[string]bool{\n")
-	for plat, hasCgo := range cgoEnabled {
-		if hasCgo {
-			fmt.Fprintf(&buf, "\t%q: true,\n", plat)
-		}
+	for _, plat := range list {
+		fmt.Fprintf(&buf, "\t%q: true,\n", plat)
 	}
 	fmt.Fprintf(&buf, "}")