[dev.link] cmd/link: migrate symbol's Value and Align when converting to sym.Symbols

Currently, in LoadFull we migrate a symbol's Value to sym.Symbol
only for external symbols. And symbol's Align is not migrated at
all. As we move LoadFull forward, there are already places where
we set symbol's Value and Align (e.g. in doelf). Migrate them
correctly.

Currently I think we only set them on external symbols, but as
we move forward I think we'll need to set them on Go symbols as
well.

Change-Id: I63e97e38fc08b653ba9faefe15697944faf21bed
Reviewed-on: https://go-review.googlesource.com/c/go/+/225658


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