cmd/dist: use a copy of platform.BuildModeSupported
The dist tool already includes a similar duplicate of BuildModeSupported.
Replace it with an exact copy, to make it easier to maintain going forward.
Change-Id: Id14a6c5a48f92d843e02218d87cc62c6b001923b
Reviewed-on: https://go-review.googlesource.com/c/go/+/467495 Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>