This refactoring is in preparation for handling auxv differently in
Android shared libraries.

Updates #18041.

Change-Id: If0458a309f9c804e7abd0a58b5a224d89f8da257
Reviewed-on: https://go-review.googlesource.com/33651
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>