cmd/go: import runtime/cgo when externally linking
Restore CL 477195, which was reverted in CL 477795.
This version includes CL 477397, which fixes the test problems
with CL 477195. CL 477397 was not submitted because it had an
unrelated failure on darwin-amd64. That failure is fixed by CL 477736.
Fixes #31544
Change-Id: I3a2258cd0ca295cede3511ab212e56fd0114f94a
Reviewed-on: https://go-review.googlesource.com/c/go/+/477839 Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>