runtime: use unsafe.Slice in itabInit

runtime: use unsafe.Slice in itabInit

As opposed to old style (*[1 << 16]T(p)[:n:n].

Change-Id: I6836f87741a2e2f6cef34f87cc839d15e309cc67
Reviewed-on: https://go-review.googlesource.com/c/go/+/726401
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>