rst: Don't allocate page for child stack (v2)

When clone-ing kids we can set their stack on current, as
it will anyway be COW-ed later. One thing to note -- we do
need to reserve some space on the stack for glibc's arguments
and retcode allocation. 128 bytes should be enough for 16
pointers while clone has 5 arguments.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>