When building test binaries, we build one archive with all of the test
sources and a second archive with the generated test package main and
link them together. If the test sources are themselves in package main
and the test was compiled with non-default compiler flags, then both
archives will contain a go.cuinfo.producer.main symbol, leading to a
duplicate symbol failure.

This has been causing test build failures on darwin-arm-a1428ios,
darwin-arm64-a1549ios, linux-amd64-noopt, android-arm-wiko-fever, and
android-arm64-wiko-fever since CL 71430 added this symbol. This CL
should fix the build.

Change-Id: I69051c846e7c0d97395a865a361cae07f411f9ad
Reviewed-on: https://go-review.googlesource.com/71771
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>