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: Ide5ad6fcec9cede99e9b21c4548929b4ba1f4185

diff --cc src/cmd/compile/internal/gc/iimport.go
Simple merge

diff --cc src/cmd/internal/obj/link.go
Simple merge