[dev.link] all: merge branch 'master' into dev.link

[dev.link] all: merge branch 'master' into dev.link

It has been a while we have not done this.

Merge conflict resolution:
- deleted/rewritten code modified on master
  - CL 214286, ported in CL 217317
    (cmd/internal/obj/objfile.go)
  - CL 210678, it already includes a fix to new code
    (cmd/link/internal/ld/deadcode.go)
  - CL 209317, applied in this CL
    (cmd/link/internal/loadelf/ldelf.go)

Change-Id: Ie927ea6a1d69ce49e8d03e56148cb2725e377876