go/build: enable cgo by default on iOS

Otherwise misc/cgo/test won't be tested on iOS.

Change-Id: I7ee78c825b0bb092c7a8b2c2ece5a6eda2f6cf95
Reviewed-on: https://go-review.googlesource.com/9643Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/build.go

@@ -259,6 +259,8 @@ var Default Context = defaultContext()
 var cgoEnabled = map[string]bool{
 	"darwin/386":      true,
 	"darwin/amd64":    true,
+	"darwin/arm":      true,
+	"darwin/arm64":    true,
 	"dragonfly/amd64": true,
 	"freebsd/386":     true,
 	"freebsd/amd64":   true,