[dev.link] cmd/link: fix end-of-block padding

Make sure we write the entire address range we are asked to write,
with no holes between the blocks or at the end.

Should fix NetBSD build.

Change-Id: I13b1f551377cbc4bcde3650417ac95cba62ff807
Reviewed-on: https://go-review.googlesource.com/c/go/+/226617


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