cmd/cgo: only write _cgo_flags for gccgo

cmd/cgo: only write _cgo_flags for gccgo

We only use it for gccgo. Also only write out LDFLAGS,
as that is all that cmd/go uses.

Fixes #60642

Change-Id: I6ccc419a17a433583d9868dd63aa7ec41c2b22c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/524556
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>