cmd/go, cmd/link: support buildmode=pie on darwin/amd64

[hirochachacha]

I finished some investigations. I'd like to handle this if no one has a CL.

[odeke-em]

All yours, and thank you @hirochachacha! I'll mark this for Go1.10.

[gopherbot]

Change https://golang.org/cl/54790 mentions this issue: cmd/go, cmd/link: enable buildmode=pie on darwin/amd64