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

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>