cmd/go: enable upx compression test on linux/386

We have a cmd/go test ensuring that upx (an executable
packer/compressor) works on linux/amd64 Go binaries.

The linux-386-sid builder is built from the same dockerfile as the
linux-amd64-sid builder, so upx should also already be available on
the former. Since upx support 386 executables, we can enable the upx
test for GOARCH=386.

Updates #16706

Change-Id: I94e19ff1001de83a0386754a5104a377c72fb221
Reviewed-on: https://go-review.googlesource.com/110817


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>