xtensa: pad IRQ stack frame to be 16 bytes aligned

Xtensa ISA mentions that stack always needs to be aligned on
16 bytes. So we need to pad the stack frame to be also 16 bytes
aligned when dealing with interrupts. Or else the stack would
not be 16 bytes aligned when we add the stack frame to stack
during interrupt handling.

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