cmd/gomobile: init darwin/arm{,64} concurrently

Drops the time for `gomobile init` on my machine from 46s to 30s.

Change-Id: Iab89f2e1d1cee4b414dc2fb1c735b4167aef9036
Reviewed-on: https://go-review.googlesource.com/12641
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

cmd/gomobile/bind_iosapp.go

@@ -44,7 +44,6 @@ func goIOSBind(pkg *build.Package) error {
 
 	cmd := exec.Command("xcrun", "lipo", "-create")
 
-	// TODO(crawshaw): Build in parallel.
 	for _, env := range [][]string{darwinArmEnv, darwinArm64Env, darwinAmd64Env} {
 		arch := archClang(getenv(env, "GOARCH"))
 		path, err := goIOSBindArchive(name, mainFile, env)

cmd/gomobile/build.go

@@ -203,9 +203,12 @@ func init() {
 }
 
 func goBuild(src string, env []string, args ...string) error {
+	// The -p flag is to speed up darwin/arm builds.
+	// Remove when golang.org/issue/10477 is resolved.
 	cmd := exec.Command(
 		"go",
 		"build",
+		fmt.Sprintf("-p=%d", runtime.NumCPU()),
 		"-pkgdir="+pkgdir(env),
 		"-tags="+strconv.Quote(strings.Join(ctx.BuildTags, ",")),
 	)

cmd/gomobile/init.go

@@ -168,7 +168,9 @@ func installStd(env []string, args ...string) error {
 		fmt.Fprintf(os.Stderr, "\n# Building standard library for %s/%s.\n", tOS, tArch)
 	}
 
-	cmd := exec.Command("go", "install", "-pkgdir="+pkgdir(env))
+	// The -p flag is to speed up darwin/arm builds.
+	// Remove when golang.org/issue/10477 is resolved.
+	cmd := exec.Command("go", "install", fmt.Sprintf("-p=%d", runtime.NumCPU()), "-pkgdir="+pkgdir(env))
 	cmd.Args = append(cmd.Args, args...)
 	if buildV {
 		cmd.Args = append(cmd.Args, "-v")