cmd/link: use sym.Symbol in addpersrc

addpersrc is called very late, after we have converted to
sym.Symbols and various fields in loader representation have been
dropped. Use the Symbol representation there.

Fixes #39658.

Change-Id: I616e838655b6f01554644171317e2cc5cefabf39
Reviewed-on: https://go-review.googlesource.com/c/go/+/238779


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>