cpu: darwin/arm64 feature detection

The ARM64 flags were not being correctly set for Mac M series laptops.

Fixes golang/go#43046

Change-Id: I1d884dccd3f3a0a010ad7babbab26b73731ab583
Reviewed-on: https://go-review.googlesource.com/c/sys/+/737260
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>