fix arm stack alignment

[crabtw]

This fixes segmentation fault of new rt tests.
For example

use core::rt::test::*;
use core::rt::comm::*;
use core::cell::Cell;
fn main() {
do run_in_newsched_task {
let (port, chan) = oneshot::<~int>();
let port_cell = Cell(port);
do spawntask_immediately {
assert!(port_cell.take().recv() == ~10);
}
chan.send(~10);
}
}

[brson]

Thanks!