cmd/6a, cmd/6l: add PCLMULQDQ instruction.

This Intel instruction implements multiplication in binary fields.

R=golang-dev, minux.ma, dave, rsc
CC=golang-dev
https://golang.org/cl/10428043

src/cmd/6a/lex.c

@@ -1019,6 +1019,7 @@ struct
 	"AESKEYGENASSIST", LTYPEX, AAESKEYGENASSIST,
 	"PSHUFD",	LTYPEX, APSHUFD,
 	"USEFIELD",	LTYPEN, AUSEFIELD,
+	"PCLMULQDQ",	LTYPEX, APCLMULQDQ,
 
 	0
 };

src/cmd/6l/6.out.h

@@ -766,6 +766,8 @@ enum	as
 	ANPTRS,
 	APTRS,
 
+	APCLMULQDQ,
+
 	ALAST
 };
 

src/cmd/6l/optab.c

@@ -1344,6 +1344,8 @@ Optab optab[] =
 	{ ANPTRS },
 	{ APTRS },
 
+	{ APCLMULQDQ,	yxshuf,	Pq, 0x3a,0x44,0 },
+
 	{ AEND },
 	0
 };