cmd/cgo/internal/testshared: log stdout when go command fails
Understanding a failure in the goCmd call is difficult because the
important information might be in the stdout instead of stderr.
Change-Id: Icf42974679103c69016129fe2ebb15d5a0a3b51a
Reviewed-on: https://go-review.googlesource.com/c/go/+/659456
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>