cmd: merge branch 'dev.link' into master

In the dev.link branch we continued developing the new object
file format support and the linker improvements described in
https://golang.org/s/better-linker . Since the last merge, more
progress has been made to improve the new linker.

This is a clean merge.

Change-Id: I57c510b651a39354d78478a9a4499f770eef2eb1