cmd/go: don't collect package CGOLDFLAGS when using gccgo

They are already collected via cmd/cgo.

The gccgo_link_c test is tweaked to do real linking as with this
change the cgo ldflags are not fully reflected in go build -n output,
since they now only come from the built archive.

This is a backport of https://go.dev/cl/497117 from the main repo.

For golang/go#60287

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/511675