The reason the 386 trybot was happy but 'GOARCH=386 go test cmd/vet'
was not is that CgoEnabled defaults to false in a cross build;
I have no idea why. Now we ask the go command for the effective
value so that the test works in both cases.

Also, remove stale comment.

Fixes #28829

Change-Id: I1210af34da6986f47924059de5c1f08b2824ace9
Reviewed-on: https://go-review.googlesource.com/c/149958
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>