runtime: check for heap allocations inside interrupts

This is unsafe and should never be done.

src/runtime/runtime_fe310.go

@@ -25,6 +25,12 @@ func main() {
 	// Zero the threshold value to allow all priorities of interrupts.
 	sifive.PLIC.THRESHOLD.Set(0)
 
+	// Zero MCAUSE, which is set to the reset reason on reset. It must be zeroed
+	// to make interrupt.In() work.
+	// This would also be a good time to save the reset reason, but that hasn't
+	// been implemented yet.
+	riscv.MCAUSE.Set(0)
+
 	// Set the interrupt address.
 	// Note that this address must be aligned specially, otherwise the MODE bits
 	// of MTVEC won't be zero.
@@ -73,6 +79,10 @@ func handleInterrupt() {
 		// misaligned loads). However, for now we'll just print a fatal error.
 		handleException(code)
 	}
+
+	// Zero MCAUSE so that it can later be used to see whether we're in an
+	// interrupt or not.
+	riscv.MCAUSE.Set(0)
 }
 
 // initPeripherals configures periperhals the way the runtime expects them.