cmd/go, cmd/link: enable buildmode=pie on darwin/amd64
Change some configurations to enable the feature. Also add the test.
This CL doesn't include internal linking support which is tentatively
disabled due to #18968. We could do that another day.
Fixes #21220
Change-Id: I601d2d78446d36332acc70be0d5b9461ac635208
Reviewed-on: https://go-review.googlesource.com/54790 Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>