cmd/asm: reject avx512 .Z instructions without a mask register

cmd/asm: reject avx512 .Z instructions without a mask register

Zeroing requires a non-K0 mask register be specified.
(gcc enforces this when assembling.)

The non-K0 restriction is already handled by the Yknot0 restriction.
But if the mask register is missing altogether, we misassemble the
instruction.

Fixes #57952

Not sure if this is really worth mentioning in the release notes,
but just in case I'll mark it.
RELNOTE=yes

Change-Id: I8f05d3155503f1f16d1b5ab9d67686fe5b64dfea
Reviewed-on: https://go-review.googlesource.com/c/go/+/463229
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Илья Токарь <tocarip@gmail.com>
Reviewed-by: Iskander Sharipov <quasilyte@gmail.com>