cpu: add runtime import in cpu_darwin_arm64_other.go

It was missed in CL 737260.

For golang/go#43046.

Change-Id: I05194333831ad04e43e22b7b34362fe01f50c000
Reviewed-on: https://go-review.googlesource.com/c/sys/+/759160
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Egon Elbre <egonelbre@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

Author: dmitshur

cpu/cpu_darwin_arm64_other.go

@@ -6,6 +6,8 @@
 
 package cpu
 
+import "runtime"
+
 func doinit() {
 	setMinimalFeatures()