[dev.link] cmd/asm, cmd/compile: add back newobj flag

Add back the newobj flag, renamed to go115newobj, for feature
gating. The flag defaults to true.

This essentially reverts CL 206398 as well as CL 220060.

The old object format isn't working yet. Will fix in followup CLs.

Change-Id: I1ace2a9cbb1a322d2266972670d27bda4e24adbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/224623
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

@@ -381,6 +381,7 @@ type LSym struct {
 	Type objabi.SymKind
 	Attribute
 
+	RefIdx int // Index of this symbol in the symbol reference list.
 	Size   int64
 	Gotype *LSym
 	P      []byte
@@ -390,7 +391,7 @@ type LSym struct {
 
 	Pkg    string
 	PkgIdx int32
-	SymIdx int32
+	SymIdx int32 // TODO: replace RefIdx
 }
 
 // A FuncInfo contains extra fields for STEXT symbols.
@@ -651,6 +652,7 @@ type Link struct {
 	Flag_linkshared    bool
 	Flag_optimize      bool
 	Flag_locationlists bool
+	Flag_go115newobj   bool // use new object file format
 	Bso                *bufio.Writer
 	Pathname           string
 	hashmu             sync.Mutex       // protects hash, funchash