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

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 #37610
Updates #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>