We're going to use this free list for other types of manually-managed
memory in the heap.

For #19325.

Change-Id: Ib7e682295133eabfddf3a84f44db43d937bfdd9c
Reviewed-on: https://go-review.googlesource.com/38575
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>