cmd/compile: use runtime hash func for known types

cmd/compile: use runtime hash func for known types

Those functions are defined in package runtime already, so just use them
instead of creating ONAME nodes with nil Func.

Change-Id: If29814a5254793c578c15b70f9c194b7414911d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/436959
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>