cmd/ld: don't generate DW_AT_type attr for unsafe.Pointer to match gcc behavior

gcc generates only attr DW_AT_byte_size for DW_TAG_pointer_type of "void *",
but we used to also generate DW_AT_type pointing to imaginary unspecified
type "void", which confuses some gdb.
This change makes old Apple gdb 6.x (specifically, Apple version gdb-1515)
accepts our binary without issue like this:
(gdb) b 'main.main'
Die: DW_TAG_unspecified_type (abbrev = 10, offset = 47079)
    has children: FALSE
    attributes:
        DW_AT_name (DW_FORM_string) string: "void"
Dwarf Error: Cannot find type of die [in module /Users/minux/go/go2.hg/bin/go]

Special thanks to Russ Cox for pointing out the problem in comment #6 of
CL 7891044.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7744051

src/cmd/ld/dwarf.c

@@ -154,6 +154,7 @@ enum
 	DW_ABRV_IFACETYPE,
 	DW_ABRV_MAPTYPE,
 	DW_ABRV_PTRTYPE,
+	DW_ABRV_BARE_PTRTYPE, // only for void*, no DW_AT_type attr to please gdb 6.
 	DW_ABRV_SLICETYPE,
 	DW_ABRV_STRINGTYPE,
 	DW_ABRV_STRUCTTYPE,
@@ -307,6 +308,12 @@ static struct DWAbbrev {
 		DW_AT_type,	DW_FORM_ref_addr,
 		0, 0
 	},
+	/* BARE_PTRTYPE */
+	{
+		DW_TAG_pointer_type, DW_CHILDREN_no,
+		DW_AT_name,	DW_FORM_string,
+		0, 0
+	},
 
 	/* SLICETYPE */
 	{
@@ -717,7 +724,7 @@ putattrs(int abbrev, DWAttr* attr)
 				attrs[af->attr]->value,
 				attrs[af->attr]->data);
 		else
-			putattr(abbrev, af->form, 0, 0, 0);
+			putattr(abbrev, af->form, 0, 0, nil);
 }
 
 static void putdie(DWDie* die);
@@ -1009,8 +1016,7 @@ defgotype(Sym *gotype)
 		break;
 
 	case KindUnsafePointer:
-		die = newdie(&dwtypes, DW_ABRV_PTRTYPE, name);
-		newrefattr(die, DW_AT_type, find(&dwtypes, "void"));
+		die = newdie(&dwtypes, DW_ABRV_BARE_PTRTYPE, name);
 		break;
 
 	default:
@@ -2126,8 +2132,7 @@ dwarfemitdebugsections(void)
 	// Some types that must exist to define other ones.
 	newdie(&dwtypes, DW_ABRV_NULLTYPE, "<unspecified>");
 	newdie(&dwtypes, DW_ABRV_NULLTYPE, "void");
-	newrefattr(newdie(&dwtypes, DW_ABRV_PTRTYPE, "unsafe.Pointer"),
-		DW_AT_type, find(&dwtypes, "void"));
+	newdie(&dwtypes, DW_ABRV_BARE_PTRTYPE, "unsafe.Pointer");
 	die = newdie(&dwtypes, DW_ABRV_BASETYPE, "uintptr");  // needed for array size
 	newattr(die, DW_AT_encoding,  DW_CLS_CONSTANT, DW_ATE_unsigned, 0);
 	newattr(die, DW_AT_byte_size, DW_CLS_CONSTANT, PtrSize, 0);