cmd/asm: add generated test of amd64 instruction encodings

cmd/asm: add generated test of amd64 instruction encodings

Generated by x86test, from https://golang.org/cl/18842
(still in progress).

The commented out lines are either missing or misspelled
or incorrectly handled instructions.

For #4816, #8037, #13822, #14068, #14069.

Change-Id: If309310c97d9d2a3c71fc64c51d4a957e9076ab7
Reviewed-on: https://go-review.googlesource.com/18850
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>