6l: fix emit windows dwarf sections

R=alex.brainman, rsc
CC=golang-dev, vcc.163
https://golang.org/cl/4532069

src/cmd/6l/asm.c

@@ -829,6 +829,7 @@ asmb(void)
 			}
 			break;
 		case Hwindows:
+			seek(cout, symo, 0);
 			if(debug['v'])
 			       Bprint(&bso, "%5.2f dwarf\n", cputime());