Xtensa port: Fixed memory corruption in interrupt handler exit function.

The a3 register was supposed to hold the _thread pointer but it seems that it
does not in all cases. Safe to restore it from _kernel structure.

Change-Id: Ie2ff6c3faf0fe70de4c5877ab59433d0c165145b
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>
1 file changed