8a, 8l: implement support for RDTSC instruction.

        Also modify runtime/asm_386.s to use it.

R=rsc
CC=golang-dev
https://golang.org/cl/5634043

src/cmd/8a/lex.c

@@ -445,6 +445,7 @@ struct
 	"RCRB",		LTYPE3,	ARCRB,
 	"RCRL",		LTYPE3,	ARCRL,
 	"RCRW",		LTYPE3,	ARCRW,
+	"RDTSC",	LTYPE0,	ARDTSC,
 	"REP",		LTYPE0,	AREP,
 	"REPN",		LTYPE0,	AREPN,
 	"RET",		LTYPE0,	ARET,

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

@@ -396,6 +396,8 @@ enum	as
 	ACMPXCHGW,
 	ACMPXCHG8B,
 
+	ARDTSC,
+
 	AXADDB,
 	AXADDL,
 	AXADDW,

src/cmd/8l/optab.c

@@ -707,6 +707,8 @@ Optab optab[] =
 	{ ACMPXCHGW,	yrl_ml,	Pm, 0xb1 },
 	{ ACMPXCHG8B,	yscond,	Pm, 0xc7,(01) },
 
+	{ ARDTSC,	ynone,	Pm, 0x31 },
+
 	{ AXADDB,	yrb_mb,	Pb, 0x0f,0xc0 },
 	{ AXADDL,	yrl_ml,	Pm, 0xc1 },
 	{ AXADDW,	yrl_ml,	Pe, 0x0f,0xc1 },

src/pkg/runtime/asm_386.s

@@ -527,7 +527,7 @@ TEXT runtime·getcallersp(SB), 7, $0
 // int64 runtime·cputicks(void), so really
 // void runtime·cputicks(int64 *ticks)
 TEXT runtime·cputicks(SB),7,$0
-	BYTE	$0x0F; BYTE $0x31;     // RDTSC; not supported by 8a
+	RDTSC
 	MOVL	ret+0(FP), DI
 	MOVL	AX, 0(DI)
 	MOVL	DX, 4(DI)