cpu: disable AVX+ on netbsd/amd64

netbsd's signal handler corrupts bits 128 and higher
in the AVX registers.

Update golang/go#80285

Change-Id: I6f486795dede81089f5ead305d10692c421d9566
Reviewed-on: https://go-review.googlesource.com/c/sys/+/797900
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Keith Randall <khr@golang.org>