arch/xtensa: Fix a race condition in xtensa_sig_deliver for SMP

After the signals have been delivered, the local irqs need to be
disabled until the context switch. But just calling
leave_critical_section(regs[REG_PS]) will enable them if they were
enabled in the context.

Any interrupt on this cpu before xtensa_context_restore() would set
the rtcb->xcp.regs to NULL, causing a crash in context restore.

Fix this by calling leave_critical_section(up_irq_save()) instead.

Signed-off-by: Jukka Laitinen <jukka.laitinen@tii.ae>

arch/xtensa/src/common/xtensa_sigdeliver.c

@@ -154,12 +154,11 @@ retry:
    */
 
   board_autoled_off(LED_SIGNAL);
+
 #ifdef CONFIG_SMP
   /* We need to keep the IRQ lock until task switching */
 
-  rtcb->irqcount++;
-  leave_critical_section((regs[REG_PS]));
-  rtcb->irqcount--;
+  leave_critical_section(up_irq_save());
 #endif
 
   rtcb->xcp.regs = rtcb->xcp.saved_regs;