ARM 4Kstacks: Add support for 4K kernel stacks to ARM

This saves 4K per kernel thread and user-space process.
For systems with lots of threads, this can save
100s of K of memory.

Change-Id: I877be9c8f7225f9395a5d5afd8ed3c638a1e1b2e
Signed-off-by: Tim Bird <tim.bird@am.sony.com>
Signed-off-by: Iliyan Malchev <malchev@google.com>