[dev.link] cmd/link: simplify asmb2

[dev.link] cmd/link: simplify asmb2

Move lots of the binary-file format specific pieces into their
appropriate places. Similarly rescope some variables to just ld.

Change-Id: I74bc6d8aba58f5ac86e6579be1fcb356c4636825
Reviewed-on: https://go-review.googlesource.com/c/go/+/235278
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>