Work around a Mac OS libc bug.
Work around a Mac OS libc bug.

If you're dismantling the alternate signal stack the ss_size field should be
ignored, but isn't on Mac OS.

Change-Id: Id6e93ed5089e375f0bd84a68efee36c34462cdf0

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst