[dev.link] cmd/link: move the wavefront past addexport()

Reorganize the linker phase ordering so that addexport() runs before
loadlibfull. In previous CLs addexport() was changed to use loader
APIs but then copy back its work into sym.Symbol, so this change
removes the copying/shim code in question.

Change-Id: I17314a90007909e6242ee00e26393f3e4a02cf25
Reviewed-on: https://go-review.googlesource.com/c/go/+/226362


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