What version of Go are you using (go version)?

$ go version
go version devel +b4ea672009 Mon Sep 21 01:30:48 2020 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes. And with Go 1.14 (but not Go 1.13)

What operating system and processor architecture are you using (go env)?

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/austin/.cache/go-build"
GOENV="/home/austin/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/austin/r/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/austin/r/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/austin/go.dev"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/austin/go.dev/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/austin/go.dev/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build542776898=/tmp/go-build -gno-record-gcc-switches"

What did you do?

While using Go 1.4 as the bootstrap compiler:
GOOS=linux GOARCH=arm64 ./make.bash

What did you expect to see?

Logs from a successful cross-compilation.

What did you see instead?

Before the successful cross-compilation, the following scary-sounding warning:
2020/09/21 09:05:27 unsupported GOARCH arm64

This output comes from make.bash invoking go version on the bootstrap compiler with the GOOS and GOARCH provided by the user. Since I'm using 1.4 as a bootstrap and it didn't support GOARCH=arm64 (or many other GOARCHes), the bootstrap compiler complains.

It looks like this was introduced in CL 204757 as a simple debugging aid to print the bootstrap compiler's version in the make.bash log. Probably the simplest thing to do is just clear GOOS and GOARCH on the go version invocation.

/cc @bradfitz @ianlancetaylor