arm: Generate privileged stacks

This patch adds the generation and incorporation of privileged stack
regions that are used by ARM user mode threads.  This patch adds the
infrastructure for privileged stacks.  Later patches will utilize the
generated stacks and helper functions.

Signed-off-by: Chunlin Han <chunlin.han@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>