runtime: fix build for non-split-stack systems

Change memory allocation accounting for stacks to use stacks_sys,
since that seems to be what it is for.

Change-Id: I9875af55e00f20e629aba2b321a3f12071e95fc6
Reviewed-on: https://go-review.googlesource.com/43297
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed