RISC-V,PLIC: Cannot mask and unmask claimed IRQ

While masking and unmasking IRQs seems to work for some IRQ sources,
there are IRQ sources where masking an IRQ during a claim causes no
more IRQs to be raised after the IRQ is unmasked. This change
explicitly follows the claim->acknowledgement procedure that the
PLIC expects.

src/object/interrupt.c

@@ -139,7 +139,11 @@ exception_t decodeIRQHandlerInvocation(word_t invLabel, irq_t irq,
 
 void invokeIRQHandler_AckIRQ(irq_t irq)
 {
+#ifdef CONFIG_ARCH_RISCV
+    plic_complete_claim(irq);
+#else
     maskInterrupt(false, irq);
+#endif
 }
 
 void invokeIRQHandler_SetIRQHandler(irq_t irq, cap_t cap, cte_t *slot)
@@ -202,7 +206,9 @@ void handleInterrupt(irq_t irq)
             printf("Undelivered IRQ: %d\n", (int)irq);
 #endif
         }
+#ifndef CONFIG_ARCH_RISCV
         maskInterrupt(true, irq);
+#endif
         break;
     }