Introduce an incomplete map in dist alongside cgoEnabled and filter out
the incomplete ports in 'dist list'.

Fixes #28944

Change-Id: I15aae56aec570e1cd9e28906900cd5ba0db77811
Reviewed-on: https://go-review.googlesource.com/c/155839
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>