Currently, we don't allow user to override xcodebuild args, we may introdude a new go build flag such as `xcodebuildargs` to pass additional flags. Updates #11043. Change-Id: I776b332c2af3d9640a7c5f23ab7bc7b19a5e408a Reviewed-on: https://go-review.googlesource.com/10990 Reviewed-by: David Crawshaw <crawshaw@golang.org>