cmd/internal/obj/x86: fix adcb r/mem8,reg8 encoding

Taken from the Intel Software Development Manual (of course, in the line
below it's ADC DST, SRC; The opposite of the commit subject).

  12 /r		ADC r8, r/m8

We need 0x12 for the corresponding ytab line, not 0x10.

  {Ymb, Ynone, Yrb, Zm_r, 1},

Updates #14069

Change-Id: Id37cbd0c581c9988c2de355efa908956278e2189
Reviewed-on: https://go-review.googlesource.com/41857Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -80,10 +80,10 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	ADCQ (R11), DX                          // 491313
 	ADCQ (BX), R11                          // 4c131b
 	ADCQ (R11), R11                         // 4d131b
-	//TODO: ADCB (BX), DL                   // 1213
-	//TODO: ADCB (R11), DL                  // 411213
-	//TODO: ADCB (BX), R11                  // 44121b
-	//TODO: ADCB (R11), R11                 // 45121b
+	ADCB (BX), DL                           // 1213
+	ADCB (R11), DL                          // 411213
+	ADCB (BX), R11                          // 44121b
+	ADCB (R11), R11                         // 45121b
 	//TODO: ADCXL (BX), DX                  // 660f38f613
 	//TODO: ADCXL (R11), DX                 // 66410f38f613
 	//TODO: ADCXL DX, DX                    // 660f38f6d2

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

@@ -945,7 +945,7 @@ var optab =
 	{AAAD, ynone, P32, [23]uint8{0xd5, 0x0a}},
 	{AAAM, ynone, P32, [23]uint8{0xd4, 0x0a}},
 	{AAAS, ynone, P32, [23]uint8{0x3f}},
-	{AADCB, yxorb, Pb, [23]uint8{0x14, 0x80, 02, 0x10, 0x10}},
+	{AADCB, yxorb, Pb, [23]uint8{0x14, 0x80, 02, 0x10, 0x12}},
 	{AADCL, yaddl, Px, [23]uint8{0x83, 02, 0x15, 0x81, 02, 0x11, 0x13}},
 	{AADCQ, yaddl, Pw, [23]uint8{0x83, 02, 0x15, 0x81, 02, 0x11, 0x13}},
 	{AADCW, yaddl, Pe, [23]uint8{0x83, 02, 0x15, 0x81, 02, 0x11, 0x13}},