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

src/cmd/internal/obj/link.go

@@ -33,6 +33,7 @@ package obj
 import (
 	"bufio"
 	"cmd/internal/dwarf"
+	"cmd/internal/goobj2"
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
@@ -666,7 +667,7 @@ type Link struct {
 	PosTable           src.PosTable
 	InlTree            InlTree // global inlining tree used by gc/inl.go
 	DwFixups           *DwarfFixupTable
-	Imports            []string
+	Imports            []goobj2.ImportedPkg
 	DiagFunc           func(string, ...interface{})
 	DiagFlush          func()
 	DebugInfo          func(fn *LSym, info *LSym, curfn interface{}) ([]dwarf.Scope, dwarf.InlCalls) // if non-nil, curfn is a *gc.Node
@@ -699,6 +700,8 @@ type Link struct {
 	defs       []*LSym // list of defined symbols in the current package
 	nonpkgdefs []*LSym // list of defined non-package symbols
 	nonpkgrefs []*LSym // list of referenced non-package symbols
+
+	Fingerprint goobj2.FingerprintType // fingerprint of symbol indices, to catch index mismatch
 }
 
 func (ctxt *Link) Diag(format string, args ...interface{}) {