cgo: use debug data section for ELF

When generating enums use the debug data section instead of the
DWARF debug info, if it is available in the ELF file. This allows
mkerrors.sh to work correctly on OpenBSD/386 and NetBSD/386.

Fixes #2470.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6495090

src/cmd/cgo/gcc.go

@@ -616,15 +616,16 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 			n.FuncType = conv.FuncType(f, pos)
 		} else {
 			n.Type = conv.Type(types[i], pos)
-			if enums[i] != 0 && n.Type.EnumValues != nil {
+			// Prefer debug data over DWARF debug output, if we have it.
+			if n.Kind == "const" && i < len(enumVal) {
+				n.Const = fmt.Sprintf("%#x", enumVal[i])
+			} else if enums[i] != 0 && n.Type.EnumValues != nil {
 				k := fmt.Sprintf("__cgo_enum__%d", i)
 				n.Kind = "const"
 				n.Const = fmt.Sprintf("%#x", n.Type.EnumValues[k])
 				// Remove injected enum to ensure the value will deep-compare
 				// equally in future loads of the same constant.
 				delete(n.Type.EnumValues, k)
-			} else if n.Kind == "const" && i < len(enumVal) {
-				n.Const = fmt.Sprintf("%#x", enumVal[i])
 			}
 		}
 	}
@@ -802,17 +803,35 @@ func (p *Package) gccDebug(stdin []byte) (*dwarf.Data, binary.ByteOrder, []byte)
 		return d, f.ByteOrder, data
 	}
 
-	// Can skip debug data block in ELF and PE for now.
-	// The DWARF information is complete.
-
 	if f, err := elf.Open(gccTmp()); err == nil {
 		d, err := f.DWARF()
 		if err != nil {
 			fatalf("cannot load DWARF output from %s: %v", gccTmp(), err)
 		}
-		return d, f.ByteOrder, nil
+		var data []byte
+		symtab, err := f.Symbols()
+		if err == nil {
+			for i := range symtab {
+				s := &symtab[i]
+				if s.Name == "__cgodebug_data" {
+					// Found it.  Now find data section.
+					if i := int(s.Section); 0 <= i && i < len(f.Sections) {
+						sect := f.Sections[i]
+						if sect.Addr <= s.Value && s.Value < sect.Addr+sect.Size {
+							if sdat, err := sect.Data(); err == nil {
+								data = sdat[s.Value-sect.Addr:]
+							}
+						}
+					}
+				}
+			}
+		}
+		return d, f.ByteOrder, data
 	}
 
+	// Can skip debug data block in PE for now.
+	// The DWARF information is complete.
+
 	if f, err := pe.Open(gccTmp()); err == nil {
 		d, err := f.DWARF()
 		if err != nil {