internal/cpu: add DIT detection on arm64

internal/cpu: add DIT detection on arm64

Add support for detecting the DIT feature on ARM64 processors. This
mirrors https://go.dev/cl/597377, but using the platform specific
semantics.

Updates #66450

Change-Id: Ia107e3e3369de7825af70823b485afe2f587358e
Reviewed-on: https://go-review.googlesource.com/c/go/+/598335
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>