[dev.link] cmd/link: rename adddynrel2 back to adddynrel

[dev.link] cmd/link: rename adddynrel2 back to adddynrel

Minor renaming cleanup to get rid of a couple of old sym.Symbol
adddynrel helpers and rename the current crop of adddynrel2
methods/functions back to adddynrel.

Change-Id: I67e76decff84d603ef765f3b6a0cd78c7f3743ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/233523
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>