High Sierra has a new commpage layout (this is issue #3188), so
we need to adjust the code to handle multiple versions of the
layout.

In preparation for this change, we rename the existing offset
macros with a prefix that identifies the commpage version they
refer to.

Updates #22037

Change-Id: Idca4b7a855a2ff6dbc434cd12453fc3194707aa8
Reviewed-on: https://go-review.googlesource.com/67331
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>