[dev.link] cmd/link: convert addexport to loader interfaces

[dev.link] cmd/link: convert addexport to loader interfaces

Convert the linker's "addexport" function to use loader interfaces
for symbol manipulation instead of *sym.Symbol.

At the moment "addexport" happens after loadlibfull (there are other
chunks of functionality in the way that haven't been converted), so
this implementation contains temporary shim code to copy back the
contents of updated loader.Sym's into the corresponding sym.Symbol.

Change-Id: I867b08e66562a2bed51560fd0be2cb64d344709c
Reviewed-on: https://go-review.googlesource.com/c/go/+/224384
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>