8a, 8l: add LFENCE, MFENCE, SFENCE

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5650076

src/cmd/8a/lex.c

@@ -663,6 +663,9 @@ struct
 	"FXTRACT",	LTYPE0, AFXTRACT,
 	"FYL2X",	LTYPE0, AFYL2X,
 	"FYL2XP1",	LTYPE0, AFYL2XP1,
+	"LFENCE",	LTYPE0, ALFENCE,
+	"MFENCE",	LTYPE0, AMFENCE,
+	"SFENCE",	LTYPE0, ASFENCE,
 
 	0
 };

src/cmd/8l/8.out.h

@@ -445,6 +445,10 @@ enum	as
 	AFCMOVNU,
 	AFCMOVUN,
 
+	ALFENCE,
+	AMFENCE,
+	ASFENCE,
+
 	ALAST
 };
 

src/cmd/8l/optab.c

@@ -755,5 +755,9 @@ Optab optab[] =
 	{ AFCMOVNU,	yfcmv,	Px, 0xdb,(03) },
 	{ AFCMOVUN,	yfcmv,	Px, 0xda,(03) },
 
+	{ ALFENCE, ynone, Pm, 0xae,0xe8 },
+	{ AMFENCE, ynone, Pm, 0xae,0xf0 },
+	{ ASFENCE, ynone, Pm, 0xae,0xf8 },
+
 	0
 };