runtime: reduce per-P memory footprint when greenteagc is disabled

There are two additional sources of memory overhead per P that come from
greenteagc. One is for ptrBuf, but on platforms other than Windows it
doesn't actually cost anything due to demand-paging (Windows also
demand-pages, but the memory is 'committed' so it still counts against
OS RSS metrics). The other is for per-sizeclass scan stats. However when
greenteagc is disabled, most of these scan stats are completely unused.

The worst-case memory overhead from these two sources is relatively
small (about 10 KiB per P), but for programs with a small memory
footprint running on a machine with a lot of cores, this can be
significant (single-digit percent).

This change does two things. First, it puts ptrBuf initialization behind
the greenteagc experiment, so now that memory is never allocated by
default. Second, it abstracts the implementation details of scan stat
collection and emission, such that we can have two different
implementations depending on the build tag. This lets us remove all the
unused stats when the greenteagc experiment is disabled, reducing the
memory overhead of the stats from ~2.6 KiB per P to 536 bytes per P.
This is enough to make the difference no longer noticable in our
benchmark suite.

Fixes #73931.

Change-Id: I4351f1cbb3f6743d8f5922d757d73442c6d6ad3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/678535
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>