[dev.link] cmd/link: add loader.Sym equivalents to ArchSyms

[dev.link] cmd/link: add loader.Sym equivalents to ArchSyms

Populate ArchSyms with loader.Sym versions of important symbols,
so as to be able to convert the code that uses ArchSyms to the
new loader interfaces.

Change-Id: If6766f7164642a9dc2e31fcf7f9280a6e95e5d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/224383
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>