liblink: fix encoding of SETcc for amd64

liblink used to encode both SETEQ BP and SETEQ CH as 0f 94 c5,
however, SETEQ BP should have used a REX prefix.

Fixes #8545.

Change-Id: Ie59c990cdd0ec506cffe4318e9ad1b48db5e57dd
Reviewed-on: https://go-review.googlesource.com/2270Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/liblink/asm6.c

@@ -1289,22 +1289,22 @@ static Optab optab[] =
 	{ ASCASL,	ynone,	Px, {0xaf} },
 	{ ASCASQ,	ynone,	Pw, {0xaf} },
 	{ ASCASW,	ynone,	Pe, {0xaf} },
-	{ ASETCC,	yscond,	Pm, {0x93,(00)} },
-	{ ASETCS,	yscond,	Pm, {0x92,(00)} },
-	{ ASETEQ,	yscond,	Pm, {0x94,(00)} },
-	{ ASETGE,	yscond,	Pm, {0x9d,(00)} },
-	{ ASETGT,	yscond,	Pm, {0x9f,(00)} },
-	{ ASETHI,	yscond,	Pm, {0x97,(00)} },
-	{ ASETLE,	yscond,	Pm, {0x9e,(00)} },
-	{ ASETLS,	yscond,	Pm, {0x96,(00)} },
-	{ ASETLT,	yscond,	Pm, {0x9c,(00)} },
-	{ ASETMI,	yscond,	Pm, {0x98,(00)} },
-	{ ASETNE,	yscond,	Pm, {0x95,(00)} },
-	{ ASETOC,	yscond,	Pm, {0x91,(00)} },
-	{ ASETOS,	yscond,	Pm, {0x90,(00)} },
-	{ ASETPC,	yscond,	Pm, {0x9b,(00)} },
-	{ ASETPL,	yscond,	Pm, {0x99,(00)} },
-	{ ASETPS,	yscond,	Pm, {0x9a,(00)} },
+	{ ASETCC,	yscond,	Pb, {0x0f,0x93,(00)} },
+	{ ASETCS,	yscond,	Pb, {0x0f,0x92,(00)} },
+	{ ASETEQ,	yscond,	Pb, {0x0f,0x94,(00)} },
+	{ ASETGE,	yscond,	Pb, {0x0f,0x9d,(00)} },
+	{ ASETGT,	yscond,	Pb, {0x0f,0x9f,(00)} },
+	{ ASETHI,	yscond,	Pb, {0x0f,0x97,(00)} },
+	{ ASETLE,	yscond,	Pb, {0x0f,0x9e,(00)} },
+	{ ASETLS,	yscond,	Pb, {0x0f,0x96,(00)} },
+	{ ASETLT,	yscond,	Pb, {0x0f,0x9c,(00)} },
+	{ ASETMI,	yscond,	Pb, {0x0f,0x98,(00)} },
+	{ ASETNE,	yscond,	Pb, {0x0f,0x95,(00)} },
+	{ ASETOC,	yscond,	Pb, {0x0f,0x91,(00)} },
+	{ ASETOS,	yscond,	Pb, {0x0f,0x90,(00)} },
+	{ ASETPC,	yscond,	Pb, {0x0f,0x9b,(00)} },
+	{ ASETPL,	yscond,	Pb, {0x0f,0x99,(00)} },
+	{ ASETPS,	yscond,	Pb, {0x0f,0x9a,(00)} },
 	{ ASHLB,	yshb,	Pb, {0xd0,(04),0xc0,(04),0xd2,(04)} },
 	{ ASHLL,	yshl,	Px, {0xd1,(04),0xc1,(04),0xd3,(04),0xd3,(04)} },
 	{ ASHLQ,	yshl,	Pw, {0xd1,(04),0xc1,(04),0xd3,(04),0xd3,(04)} },