xtensa: userspace: swap page table at end of cross stack call

The logic to swap page tables or MPU entries is moved to the end
of cross stack call, since it is still running in the interrupt
stack instead of the thread stack. The old logic was calling
the swap functions in the outgoing thread stack, which is not
desirable.

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