go/build: support building cgo packages on non intel platforms

See https://golang.org/cl/4572045/

R=adg, rsc
CC=golang-dev
https://golang.org/cl/4627041

src/pkg/go/build/build.go

@@ -331,11 +331,14 @@ func (b *build) gccLink(ofile string, ofiles ...string) {
 
 func (b *build) gccArgs(args ...string) []string {
 	// TODO(adg): HOST_CC
-	m := "-m32"
-	if b.arch == "6" {
-		m = "-m64"
+	a := []string{"gcc", "-I", b.path, "-g", "-fPIC", "-O2"}
+	switch b.arch {
+	case "8":
+		a = append(a, "-m32")
+	case "6":
+		a = append(a, "-m64")
 	}
-	return append([]string{"gcc", m, "-I", b.path, "-g", "-fPIC", "-O2"}, args...)
+	return append(a, args...)
 }
 
 func (b *build) cgo(cgofiles []string) (outGo, outObj []string) {

src/pkg/go/build/build_test.go

@@ -7,8 +7,6 @@ package build
 import (
 	"exec"
 	"path/filepath"
-	"runtime"
-	"strings"
 	"testing"
 )
 
@@ -22,11 +20,6 @@ const cmdtestOutput = "3"
 
 func TestBuild(t *testing.T) {
 	for _, pkg := range buildPkgs {
-		if runtime.GOARCH == "arm" && strings.Contains(pkg, "/cgo") {
-			// no cgo for arm, yet.
-			continue
-		}
-
 		tree := Path[0] // Goroot
 		dir := filepath.Join(tree.SrcDir(), pkg)