cmd/go: fail with nice error message on bad GOOS/GOARCH pair

Fixes #12272

Change-Id: I2115ec62ed4061084c482eb385a583a1c1909888
Reviewed-on: https://go-review.googlesource.com/22838
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

.gitignore

@@ -26,6 +26,7 @@ misc/cgo/stdio/run.out
 misc/cgo/testso/main
 src/cmd/cgo/zdefaultcc.go
 src/cmd/go/zdefaultcc.go
+src/cmd/go/zosarch.go
 src/cmd/internal/obj/zbootstrap.go
 src/go/build/zcgo.go
 src/go/doc/headscan

src/cmd/dist/build.go

@@ -464,6 +464,7 @@ var deptab = []struct {
 }{
 	{"cmd/go", []string{
 		"zdefaultcc.go",
+		"zosarch.go",
 	}},
 	{"runtime/internal/sys", []string{
 		"zversion.go",
@@ -485,6 +486,7 @@ var gentab = []struct {
 	gen        func(string, string)
 }{
 	{"zdefaultcc.go", mkzdefaultcc},
+	{"zosarch.go", mkzosarch},
 	{"zversion.go", mkzversion},
 	{"zcgo.go", mkzcgo},
 

src/cmd/dist/buildgo.go

@@ -23,9 +23,7 @@ import (
 // It is invoked to write cmd/go/zdefaultcc.go
 // but we also write cmd/cgo/zdefaultcc.go
 func mkzdefaultcc(dir, file string) {
-	var out string
-
-	out = fmt.Sprintf(
+	out := fmt.Sprintf(
 		"// auto generated by go tool dist\n"+
 			"\n"+
 			"package main\n"+
@@ -42,7 +40,16 @@ func mkzdefaultcc(dir, file string) {
 	writefile(out, file, writeSkipSame)
 }
 
-// mkzcgo writes zcgo.go for go/build package:
+// mkzcgo writes zosarch.go for cmd/go.
+func mkzosarch(dir, file string) {
+	var buf bytes.Buffer
+	buf.WriteString("// auto generated by go tool dist\n\n")
+	buf.WriteString("package main\n\n")
+	fmt.Fprintf(&buf, "var osArchSupportsCgo = %#v", cgoEnabled)
+	writefile(buf.String(), file, writeSkipSame)
+}
+
+// mkzcgo writes zcgo.go for the go/build package:
 //
 //	package build
 //  var cgoEnabled = map[string]bool{}

src/cmd/go/build.go

@@ -669,6 +669,12 @@ var (
 func init() {
 	goarch = buildContext.GOARCH
 	goos = buildContext.GOOS
+
+	if _, ok := osArchSupportsCgo[goos+"/"+goarch]; !ok {
+		fmt.Fprintf(os.Stderr, "cmd/go: unsupported GOOS/GOARCH pair %s/%s\n", goos, goarch)
+		os.Exit(2)
+	}
+
 	if goos == "windows" {
 		exeSuffix = ".exe"
 	}