runtime: use offAddr in more parts of the runtime

This change uses the new offAddr type in more parts of the runtime where
we've been implicitly switching from the default address space to a
contiguous view. The purpose of offAddr is to represent addresses in the
contiguous view of the address space, and to make direct computations
between real addresses and offset addresses impossible. This change thus
improves readability in the runtime.

Updates #35788.

Change-Id: I4e1c5fed3ed68aa12f49a42b82eb3f46aba82fc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/230718


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>