[dev.link] cmd/link: remove duplicate asmb code

[dev.link] cmd/link: remove duplicate asmb code

Lots of the architecture specific code for asmb() is very simimar. As
such, move to a common function.

Change-Id: Id1fd50ee7bfa1bc9978e3f42ad08914b04cd677b
Reviewed-on: https://go-review.googlesource.com/c/go/+/234683
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>