runtime: change lfstack support to taggedPointer

runtime: change lfstack support to taggedPointer

This is a refactoring with no change in behavior, in preparation
for future netpoll work.

For #59545

Change-Id: I493c5fd0f49f31b75787f7b5b89c544bed73f64f
Reviewed-on: https://go-review.googlesource.com/c/go/+/484836
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Orlando Labao <orlando.labao43@gmail.com>