cmd/6l: accept NOP of $x+10(SP) and of X0

Needed to link code compiled with 6c -N.

R=ken2
CC=golang-dev
https://golang.org/cl/10043044

src/cmd/6l/optab.c

@@ -42,11 +42,15 @@ uchar	ytext[] =
 };
 uchar	ynop[] =
 {
-	Ynone,	Ynone,	Zpseudo,1,
-	Ynone,	Yml,	Zpseudo,1,
-	Ynone,	Yrf,	Zpseudo,1,
-	Yml,	Ynone,	Zpseudo,1,
-	Yrf,	Ynone,	Zpseudo,1,
+	Ynone,	Ynone,	Zpseudo,0,
+	Ynone,	Yiauto,	Zpseudo,0,
+	Ynone,	Yml,	Zpseudo,0,
+	Ynone,	Yrf,	Zpseudo,0,
+	Ynone,	Yxr,	Zpseudo,0,
+	Yiauto,	Ynone,	Zpseudo,0,
+	Yml,	Ynone,	Zpseudo,0,
+	Yrf,	Ynone,	Zpseudo,0,
+	Yxr,	Ynone,	Zpseudo,1,
 	0
 };
 uchar	yxorb[] =