cmd/internal/obj/x86: use named consts for movtab Z-cases

Use 0-terminated opbyte sequences for Zlit-like movtabs instead of E=0xff.

movCodeFullPtr is unused (load full ptr is unsupported), but it should
be removed in a separate CL (if removed at all).

Passes toolstash-check.

Change-Id: I28436718d93b017153de0e50e3bcec344ea4ee05
Reviewed-on: https://go-review.googlesource.com/107076
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/asm/internal/asm/testdata/386enc.s

@@ -28,5 +28,10 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	MOVL DR0, AX // 0f21c0
 	MOVL DR6, DX // 0f21f2
 	MOVL DR7, SI // 0f21fe
+	// Test other movtab entries.
+	PUSHL SS // 16
+	PUSHL FS // 0fa0
+	POPL FS  // 0fa1
+	POPL SS  // 17
 	// End of tests.
 	RET

src/cmd/asm/internal/asm/testdata/amd64enc_extra.s

@@ -309,5 +309,10 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	MOVQ DR0, AX // 0f21c0
 	MOVQ DR6, DX // 0f21f2
 	MOVQ DR7, SI // 0f21fe
+	// Test other movtab entries.
+	PUSHQ GS // 0fa8
+	PUSHQ FS // 0fa0
+	POPQ FS  // 0fa1
+	POPQ GS  // 0fa9
 	// End of tests.
 	RET