cmd/dist: add openbsd/ppc64 as a known GOOS/GOARCH
Also map uname output containing powerpc64 to ppc64 on openbsd.
Updates #56001
Change-Id: I6a4470cb0e5d6c6940d5268a6a06d23430c7859a
Reviewed-on: https://go-review.googlesource.com/c/go/+/475635 Reviewed-by: Eric Grosse <grosse@gmail.com>
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>