ukernel: set pthread stacksize to an integer multiple of 8k

ukernel: set pthread stacksize to an integer multiple of 8k

because some pthread stacks require this to be in units of page size,
guess that page sizes will be 4k or 8k, and use the larger as a divisor

Reviewed-on: http://gerrit.openafs.org/5606
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 7ae1921b76750ef28258777cf1b233c4917663d7)

Change-Id: I273ee935db4cf6c17ca50eedab8cfd577d21a599
Reviewed-on: http://gerrit.openafs.org/5707
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>