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.
Fixes #60287
Change-Id: Id433435fe8aeb9571327bf936e52a37f400cef4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/497117
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org> Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Andrey Bokhanko <andreybokhanko@gmail.com>