xtensa: set pointer to privileged stack only if user mode

When initializing the stack at thread creation, we should not
set the pointer to privileged stack pointer yet as the thread
can be a kernel thread. Only when a thread is transitioning to
user mode, then we need to set the pointer to point to
the privileged stack. This is a purely semantic change and
should not affect any functionality.

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