[dev.link] cmd/link: convert asmb2 pass to new style on darwin

[dev.link] cmd/link: convert asmb2 pass to new style on darwin

Now we no longer do loadlibfull on darwin.

While here, remove residual darwin/386 and darwin/arm code.

Change-Id: I6efdcd81baeeca29d1fe91c4fab0cc8241a58e2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/232597
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>