cmd/asm: add amd64 CLFLUSH instruction

This is the last instruction I found missing in SSE2 set.

It does not reuse 'yprefetch' ytabs due to differences in
operands SRC/DST roles:
- PREFETCHx: ModRM:r/m(r) -> FROM
- CLFLUSH:   ModRM:r/m(w) -> TO

unaryDst map is extended accordingly.

Change-Id: I89e34ebb81cc0ee5f9ebbb1301bad417f7ee437f
Reviewed-on: https://go-review.googlesource.com/56833
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/asm/internal/asm/testdata/amd64enc.s

@@ -771,8 +771,8 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	//TODO: CLAC                            // 0f01ca
 	CLC                                     // f8
 	CLD                                     // fc
-	//TODO: CLFLUSH (BX)                    // 0fae3b
-	//TODO: CLFLUSH (R11)                   // 410fae3b
+	CLFLUSH (BX)                            // 0fae3b
+	CLFLUSH (R11)                           // 410fae3b
 	//TODO: CLFLUSHOPT (BX)                 // 660fae3b
 	//TODO: CLFLUSHOPT (R11)                // 66410fae3b
 	CLI                                     // fa

src/cmd/internal/obj/x86/a.out.go

@@ -76,6 +76,7 @@ const (
 	ABYTE
 	ACLC
 	ACLD
+	ACLFLUSH
 	ACLI
 	ACLTS
 	ACMC

src/cmd/internal/obj/x86/anames.go

@@ -38,6 +38,7 @@ var Anames = []string{
 	"BYTE",
 	"CLC",
 	"CLD",
+	"CLFLUSH",
 	"CLI",
 	"CLTS",
 	"CMC",

src/cmd/internal/obj/x86/asm6.go

@@ -551,6 +551,10 @@ var ypopl = []ytab{
 	{Ynone, Ynone, Ym, Zo_m, 2},
 }
 
+var yclflush = []ytab{
+	{Ynone, Ynone, Ym, Zo_m, 2},
+}
+
 var ybswap = []ytab{
 	{Ynone, Ynone, Yrl, Z_rp, 2},
 }
@@ -994,6 +998,7 @@ var optab =
 	{ACDQ, ynone, Px, [23]uint8{0x99}},
 	{ACLC, ynone, Px, [23]uint8{0xf8}},
 	{ACLD, ynone, Px, [23]uint8{0xfc}},
+	{ACLFLUSH, yclflush, Pm, [23]uint8{0xae, 07}},
 	{ACLI, ynone, Px, [23]uint8{0xfa}},
 	{ACLTS, ynone, Pm, [23]uint8{0x06}},
 	{ACMC, ynone, Px, [23]uint8{0xf5}},

src/cmd/internal/obj/x86/obj6.go

@@ -1183,6 +1183,7 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 var unaryDst = map[obj.As]bool{
 	ABSWAPL:    true,
 	ABSWAPQ:    true,
+	ACLFLUSH:   true,
 	ACMPXCHG8B: true,
 	ADECB:      true,
 	ADECL:      true,