cmd/go: fix not print GCCGO when it's not the default

Fixes #69994

Change-Id: I2a23e5998b7421fd5ae0fdb68303d3244361b341
Reviewed-on: https://go-review.googlesource.com/c/go/+/671635
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: qiulaidongfeng

src/cmd/go/internal/envcmd/env.go

@@ -142,7 +142,7 @@ func MkEnv() []cfg.EnvVar {
 	if work.GccgoBin != "" {
 		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoBin, Changed: true})
 	} else {
-		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoName})
+		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoName, Changed: work.GccgoChanged})
 	}
 
 	goarch, val, changed := cfg.GetArchEnv()

src/cmd/go/internal/work/gccgo.go

@@ -27,13 +27,17 @@ import (
 type gccgoToolchain struct{}
 
 var GccgoName, GccgoBin string
+var GccgoChanged bool
 var gccgoErr error
 
 func init() {
 	GccgoName = cfg.Getenv("GCCGO")
 	if GccgoName == "" {
 		GccgoName = "gccgo"
 	}
+	if GccgoName != "gccgo" {
+		GccgoChanged = true
+	}
 	GccgoBin, gccgoErr = pathcache.LookPath(GccgoName)
 }
 

src/cmd/go/testdata/script/env_changed.txt

@@ -12,6 +12,7 @@ env CGO_CFLAGS=nodefault
 env CGO_CPPFLAGS=nodefault
 env GOFIPS140=latest
 [cgo] env CGO_ENABLED=0
+env GCCGO=nodefault
 
 go env -changed
 # linux output like GOTOOLCHAIN='local'
@@ -24,6 +25,7 @@ stdout 'CGO_CFLAGS=''?nodefault''?'
 stdout 'CGO_CPPFLAGS=''?nodefault''?'
 stdout 'GOFIPS140=''?latest''?'
 [cgo] stdout 'CGO_ENABLED=''?0''?'
+stdout 'GCCGO=''?nodefault''?'
 
 go env -changed -json
 stdout '"GOTOOLCHAIN": "local"'
@@ -34,6 +36,7 @@ stdout '"CGO_CFLAGS": "nodefault"'
 stdout '"CGO_CPPFLAGS": "nodefault"'
 stdout '"GOFIPS140": "latest"'
 [cgo] stdout '"CGO_ENABLED": "0"'
+stdout '"GCCGO": "nodefault"'
 
 [GOOS:windows] env GOOS=linux
 [!GOOS:windows] env GOOS=windows