simd/archsimd: fix "go generate" command

simd/archsimd: fix "go generate" command

Correct the generate command for test helpers. There is no longer
a genfiles.go. Also correct the generated file headers to match
the current generator layout.

Change-Id: Ifb9a8c394477359020ff44290dbaabe7a2d59aca
Reviewed-on: https://go-review.googlesource.com/c/go/+/732280
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>