cmd/link: avoid duplicate DT_NEEDED entries

When adding a new library entry, ensure we record it as seen to avoid
adding duplicates of it.

Fixes #39256

Change-Id: Id309adf80c533d78fd485517c18bc9ab5f1d29fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/235257


Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>