Roll forward of 54efdc59. Better testing of the build on
darwin/amd64. There is still some variance between cmd/dist
and the Go tool for build tag handling.

Change-Id: I105669ae7f90c8c89b3839c04b182cff46be8dde
Reviewed-on: https://go-review.googlesource.com/6516
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>