[dev.link] cmd/link: run setArchSyms earlier

Add some new loader.Sym equivalents to the archSyms struct so that we
can run setArchSyms earlier in the pipeline, and add a "mode" to
setArchSyms to control whether it should create loader.Sym symbols or
their *sym.Symbol equivalents.

These change needed for a subsequent patch in which addexport() is run
earlier as well

Change-Id: I0475c9388c39f13e045dd4aa9c90eaec42624810
Reviewed-on: https://go-review.googlesource.com/c/go/+/226361


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