cpu: add support for AVX-VNNI and IFMA detection

Added detection for x86 AVX-VNNI (VEX-coded Vector Neural Network
Instructions) and AVX-IFMA (VEX-coded Integer Fused Multiply Add),
including both the base VNNI set and the Int8 extention.

Fixes golang/go#71142

Change-Id: I9e8d18b2e8bf81d5d4313a4a47fdf731fb3d44dd
GitHub-Last-Rev: 32ea443fc247f1b6e957d2b27a44909c620d2fb2
GitHub-Pull-Request: golang/sys#242
Reviewed-on: https://go-review.googlesource.com/c/sys/+/641155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>