[dev.link] cmd/internal/obj: write object file in new format

If -newobj is set, write object file in new format, which uses
indices for symbol references instead of symbol names. The file
format is described at the beginning of
cmd/internal/goobj2/objfile.go.

A new package, cmd/internal/goobj2, is introduced for reading and
writing new object files. (The package name is temporary.) It is
written in a way that trys to make the encoding as regular as
possible, and the reader and writer as symmetric as possible.

This is incomplete, and currently nothing will consume the new
object file.

Change-Id: Ifefedbf6456d760d15a9f40a28af6486c93100fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/196030
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/obj/link.go

@@ -413,6 +413,8 @@ type FuncInfo struct {
 	GCLocals     *LSym
 	GCRegs       *LSym
 	StackObjects *LSym
+
+	FuncInfoSym *LSym
 }
 
 type InlMark struct {
@@ -636,15 +638,6 @@ type Pcdata struct {
 	P []byte
 }
 
-// Package Index.
-const (
-	PkgIdxNone    = (1<<31 - 1) - iota // Non-package symbols
-	PkgIdxBuiltin                      // Predefined symbols // TODO: not used for now, we could use it for compiler-generated symbols like runtime.newobject
-	PkgIdxSelf                         // Symbols defined in the current package
-	PkgIdxInvalid = 0
-	// The index of other referenced packages starts from 1.
-)
-
 // Link holds the context for writing object code from a compiler
 // to be linker input or for reading that input into the linker.
 type Link struct {