cmd/asm: add YMM registers Y0 through Y15

Not recognized in any instructions yet, but this lets the
assembler parse them at least.

For #14068.

Change-Id: Id4f7329a969b747a867ce261b20165fab2cdcab8
Reviewed-on: https://go-review.googlesource.com/18846Reviewed-by: Rob Pike <r@golang.org>

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

@@ -864,6 +864,23 @@ const (
 	REG_X14
 	REG_X15
 
+	REG_Y0
+	REG_Y1
+	REG_Y2
+	REG_Y3
+	REG_Y4
+	REG_Y5
+	REG_Y6
+	REG_Y7
+	REG_Y8
+	REG_Y9
+	REG_Y10
+	REG_Y11
+	REG_Y12
+	REG_Y13
+	REG_Y14
+	REG_Y15
+
 	REG_CS
 	REG_SS
 	REG_DS

src/cmd/internal/obj/x86/list6.go

@@ -104,6 +104,22 @@ var Register = []string{
 	"X13",
 	"X14",
 	"X15",
+	"Y0",
+	"Y1",
+	"Y2",
+	"Y3",
+	"Y4",
+	"Y5",
+	"Y6",
+	"Y7",
+	"Y8",
+	"Y9",
+	"Y10",
+	"Y11",
+	"Y12",
+	"Y13",
+	"Y14",
+	"Y15",
 	"CS", /* [D_CS] */
 	"SS",
 	"DS",