cmd/go: import runtime/cgo when externally linking
Fixes #31544
Change-Id: Ic99875ad227876eb741e93653589310327c9c0ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/477195
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>