x86: use CONFIG_PRIVILEGED_STACK_SIZE for stack size

Instead of using CONFIG_MMU_PAGE_SIZE for size of the privileged
stack, use the actual kconfig CONFIG_PRIVILEGED_STACK_SIZE.
This allows for changing the size of privileged stack, and
also aligns to the usage of CONFIG_PRIVILEGED_STACK_SIZE.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>