xtensa: userspace: swap page table earlier in xtensa_switch

When switching page table and MPU enties, we need to call
corresponding functions via call4 which would move the register
window. So we need to do that earlier before we start saving
context into stack by manipulating stack pointer manually which
definitely would interfere with window spilling.

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