cmd/link, cmd/internal/obj: use aux symbol for global variable DWARF info

Currently, for a global variable, its debug info symbol is a named
symbol with the variable's name with a special prefix. And the
linker looks it up by name. This CL makes the debug info symbol an
aux symbol of the variable symbol.

Change-Id: I55614d0ef2af9c53eb40144ad80e09339bf3cbee
Reviewed-on: https://go-review.googlesource.com/c/go/+/490816
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/obj/link.go

@@ -465,7 +465,7 @@ type LSym struct {
 	P      []byte
 	R      []Reloc
 
-	Extra *interface{} // *FuncInfo or *FileInfo, if present
+	Extra *interface{} // *FuncInfo, *VarInfo, or *FileInfo, if present
 
 	Pkg    string
 	PkgIdx int32
@@ -537,6 +537,30 @@ func (s *LSym) Func() *FuncInfo {
 	return f
 }
 
+type VarInfo struct {
+	dwarfInfoSym *LSym
+}
+
+// NewVarInfo allocates and returns a VarInfo for LSym.
+func (s *LSym) NewVarInfo() *VarInfo {
+	if s.Extra != nil {
+		panic(fmt.Sprintf("invalid use of LSym - NewVarInfo with Extra of type %T", *s.Extra))
+	}
+	f := new(VarInfo)
+	s.Extra = new(interface{})
+	*s.Extra = f
+	return f
+}
+
+// VarInfo returns the *VarInfo associated with s, or else nil.
+func (s *LSym) VarInfo() *VarInfo {
+	if s.Extra == nil {
+		return nil
+	}
+	f, _ := (*s.Extra).(*VarInfo)
+	return f
+}
+
 // A FileInfo contains extra fields for SDATA symbols backed by files.
 // (If LSym.Extra is a *FileInfo, LSym.P == nil.)
 type FileInfo struct {