runtime/fe310: add support for PLIC interrupts

This commit adds support for software vectoring in the PLIC interrupt.
The interrupt table is created by the compiler, which leads to very
compact code while retaining the flexibility that the interrupt API
provides.

src/runtime/runtime_fe310.go

@@ -42,6 +42,10 @@ func main() {
 	// of MTVEC won't be zero.
 	riscv.MTVEC.Set(uintptr(unsafe.Pointer(&handleInterruptASM)))
 
+	// Reset the MIE register and enable external interrupts.
+	// It must be reset here because it not zeroed at startup.
+	riscv.MIE.Set(1 << 11) // bit 11 is for machine external interrupts
+
 	// Enable global interrupts now that they've been set up.
 	riscv.MSTATUS.SetBits(1 << 3) // MIE
 
@@ -68,6 +72,13 @@ func handleInterrupt() {
 			// Disable the timer, to avoid triggering the interrupt right after
 			// this interrupt returns.
 			riscv.MIE.ClearBits(1 << 7) // MTIE bit
+		case 11: // Machine external interrupt
+			// Claim this interrupt.
+			id := sifive.PLIC.CLAIM.Get()
+			// Call the interrupt handler, if any is registered for this ID.
+			callInterruptHandler(int(id))
+			// Complete this interrupt.
+			sifive.PLIC.CLAIM.Set(id)
 		}
 	} else {
 		// Topmost bit is clear, so it is an exception of some sort.
@@ -167,3 +178,7 @@ func handleException(code uint) {
 	println()
 	abort()
 }
+
+// callInterruptHandler is a compiler-generated function that calls the
+// appropriate interrupt handler for the given interrupt ID.
+func callInterruptHandler(id int)