To prevent races with the garbage collector, stack spans cannot be
reused as heap spans during a GC. We deal with this by caching stack
spans during GC and releasing them at the end of mark termination.
However, while our cache lets us reuse small stack spans, currently
large stack spans are *not* reused. This can cause significant memory
growth in programs that allocate large stacks rapidly, but grow the
heap slowly (such as in issue #13552).

Fix this by adding logic to reuse large stack spans for other stacks.

Fixes #11466.

Fixes #13552. Without this change, the program in this issue creeps to
over 1GB of memory over the course of a few hours. With this change,
it stays rock solid at around 30MB.

Change-Id: If8b2d85464aa80c96230a1990715e39aa803904f
Reviewed-on: https://go-review.googlesource.com/17814Reviewed-by: Keith Randall <khr@golang.org>