...Z_THREAD_STACK_BUFFER.

This is currently a symbolic change as Z_THREAD_STACK_BUFFER
is simply an alias to Z_KERNEL_STACK_BUFFER without userspace,
and Xtensa does not support userspace at the moment.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
(cherry picked from commit b820cde7)