xtensa: window may overflow during irq stack crossing

When crossing stack during interrupt handling, we do two call4
to pivot to the interrupt stack, with arguments to these two
call4 stashed in A6, A10 and A11. However, A4-A11 may be marked
as invalid in the register file, and accessing them would
result in window overflowing. At that point, A0 and A1 are not
setup to handle window overflows, and will result in registers
being stashed in incorrect location, resulting in incorrect
value being restored during window underflowing. So move around
the code a bit to restore A0 and A1 properly before accessing
A4-A11.

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