[dev.garbage] runtime: use s.base() everywhere it makes sense

[dev.garbage] runtime: use s.base() everywhere it makes sense

Currently we have lots of (s.start << _PageShift) and variants. We now
have an s.base() function that returns this. It's faster and more
readable, so use it.

Change-Id: I888060a9dae15ea75ca8cc1c2b31c905e71b452b
Reviewed-on: https://go-review.googlesource.com/22559
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>