cmd/ld: generate correct upper bound value for array types.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/115820043

src/cmd/ld/decodesym.c

@@ -104,7 +104,7 @@ decodetype_arrayelem(LSym *s)
 vlong
 decodetype_arraylen(LSym *s)
 {
-	return decode_inuxi(s->p + commonsize()+PtrSize, PtrSize);
+	return decode_inuxi(s->p + commonsize()+2*PtrSize, PtrSize);
 }
 
 // Type.PtrType.elem
@@ -120,6 +120,7 @@ decodetype_mapkey(LSym *s)
 {
 	return decode_reloc_sym(s, commonsize());	// 0x1c / 0x30
 }
+
 LSym*
 decodetype_mapvalue(LSym *s)
 {

src/cmd/ld/dwarf.c

@@ -992,7 +992,7 @@ defgotype(LSym *gotype)
 		s = decodetype_arrayelem(gotype);
 		newrefattr(die, DW_AT_type, defgotype(s));
 		fld = newdie(die, DW_ABRV_ARRAYRANGE, "range");
-		newattr(fld, DW_AT_upper_bound, DW_CLS_CONSTANT, decodetype_arraylen(gotype), 0);
+		newattr(fld, DW_AT_upper_bound, DW_CLS_CONSTANT, decodetype_arraylen(gotype)-1, 0); // -1: want upper bound, not count.
 		newrefattr(fld, DW_AT_type, find_or_diag(&dwtypes, "uintptr"));
 		break;