gc: Free pooled stacks of exited threads on every sweep (#64036)
When a thread has exited (e.g. a foreign thread adopted with
`jl_adopt_thread`), its pooled task stacks were only unmapped by the sweep
whose rotating `gc_stack_free_idx` happened to point at it. Every other sweep
called `small_arraylist_free` on its first stack pool, which drops the list
without unmapping the stacks in it. Those stacks were leaked, and they kept
counting towards `MAX_STACK_MAPPINGS`, after which new tasks fail to get a
stack.
Free all pooled stacks of an exited thread on every sweep instead.
Assisted-by: Claude Code (Claude Opus 5.5)
