Clean up tagged pointers a bit. I got the shifts wrong
for the weird aix case.

Change-Id: I21449fd5973f4651fd1103d3b8be9c2b9b93a490
Reviewed-on: https://go-review.googlesource.com/c/go/+/667715


Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>