cmd/dist, internal/platform: test agreement on supported build modes

cmd/dist, internal/platform: test agreement on supported build modes

This synchronizes the supported build modes between cmd/dist and
internal/platform, and adds a test to keep them in synch.

In order to do that, this has several changes to cmd/dist, and one
change to internal/platform.

If the build dashboard is green after this is submitted, we can
probably make the functions identical.

Change-Id: Ia78ce76b193399058fde79e38dd9f23818e566a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/463992
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>