[release-branch.go1.15] cmd/go: permit CGO_LDFLAGS to appear in //go:ldflag

For #42565
Fixes #42567

Change-Id: If7cf39905d124dbd54dfac6a53ee38270498efed
Reviewed-on: https://go-review.googlesource.com/c/go/+/269818


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 782cf560)
Reviewed-on: https://go-review.googlesource.com/c/go/+/270137