sandbox: allow stack mmap with prot_none

This fixes a failure that was showing up on i386 Debian hosts
with sandboxing enabled, now that cpuworker is enabled on clients.
We already had allowances for creating threads and creating stacks
in the sandbox, but prot_none (probably used for a stack guard)
was not allowed so thread creation failed.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>