When enabling DIT on ARM64, add speculation barrier instructions to
ensure that subsequent instructions are executed using the updated
DIT state.
See https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Enable-DIT-for-constant-time-cryptographic-operations
which recommends doing this. The Arm documentation for DIT doesn't tell
you to do this, but it seems prudent.
Change-Id: Idbc87b332650a77b8cb3509c11377bf5c724f3cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/726980
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>