cmd/dist: remove darwin/386, darwin/arm as valid ports

[sthagen]

This only removes the ability to build it, and removes it as a
src/buildall.bash target (which uses go tool dist list).

Now:

$ go tool dist list | grep ^darwin
darwin/amd64
darwin/arm64

After this, remaining is removing leftover port--specific code in the
tree.

Updates golang#37610
Updates golang#37611

Change-Id: I00f03b2355c2e152f75e57abd3063be243529d2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/226985
Run-TryBot: Brad Fitzpatrick bradfitz@golang.org
TryBot-Result: Gobot Gobot gobot@golang.org
Reviewed-by: Filippo Valsorda filippo@golang.org

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