test: recognize cgo build tag

This requires us to add a fake argument to issue36705.go so that the
test driver will build it with "go run" rather than "go tool compile".

Change-Id: Id08b97d898ee3e9d6c1fbb072a0a9317ed9faedd
Reviewed-on: https://go-review.googlesource.com/c/go/+/304569


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>