Add support for GOARM parameter when cross compiling

Go can cross compile to older ARM architectures (e.g. v5). In order to
do so one needs to use the GOARM parameter as documented in
https://github.com/golang/go/wiki/GoArm

build.go

@@ -193,6 +193,7 @@ func showUsage(output io.Writer) {
 	fmt.Fprintf(output, "         --enable-cgo    use CGO to link against libc\n")
 	fmt.Fprintf(output, "         --goos value    set GOOS for cross-compilation\n")
 	fmt.Fprintf(output, "         --goarch value  set GOARCH for cross-compilation\n")
+	fmt.Fprintf(output, "         --goarm value  set GOARM for cross-compilation\n")
 }
 
 func verbosePrintf(message string, args ...interface{}) {
@@ -218,13 +219,16 @@ func cleanEnv() (env []string) {
 }
 
 // build runs "go build args..." with GOPATH set to gopath.
-func build(cwd, goos, goarch, gopath string, args ...string) error {
+func build(cwd, goos, goarch, goarm, gopath string, args ...string) error {
 	a := []string{"build"}
 	a = append(a, "-asmflags", fmt.Sprintf("-trimpath=%s", gopath))
 	a = append(a, "-gcflags", fmt.Sprintf("-trimpath=%s", gopath))
 	a = append(a, args...)
 	cmd := exec.Command("go", a...)
 	cmd.Env = append(cleanEnv(), "GOPATH="+gopath, "GOARCH="+goarch, "GOOS="+goos)
+	if goarm != "" {
+		cmd.Env = append(cmd.Env, "GOARM="+goarm)
+	}
 	if !enableCGO {
 		cmd.Env = append(cmd.Env, "CGO_ENABLED=0")
 	}
@@ -397,6 +401,7 @@ func main() {
 
 	targetGOOS := runtime.GOOS
 	targetGOARCH := runtime.GOARCH
+	targetGOARM := ""
 
 	var outputFilename string
 
@@ -430,6 +435,9 @@ func main() {
 		case "--goarch":
 			skipNext = true
 			targetGOARCH = params[i+1]
+		case "--goarm":
+			skipNext = true
+			targetGOARM = params[i+1]
 		case "-h":
 			showUsage(os.Stdout)
 			return
@@ -514,7 +522,7 @@ func main() {
 		"-o", output, config.Main,
 	}
 
-	err = build(filepath.Join(gopath, "src"), targetGOOS, targetGOARCH, gopath, args...)
+	err = build(filepath.Join(gopath, "src"), targetGOOS, targetGOARCH, targetGOARM, gopath, args...)
 	if err != nil {
 		die("build failed: %v\n", err)
 	}

doc/020_installation.rst

@@ -113,6 +113,8 @@ supply the target OS and platform via the command-line options like this
 
     $ go run build.go --goos freebsd --goarch 386
 
+    $ go run build.go --goos linux --goarch arm --goarm 6
+    
 The resulting binary is statically linked and does not require any
 libraries.