runtime: check for heap allocations inside interrupts

This is unsafe and should never be done.

src/runtime/interrupt/interrupt_tinygoriscv.go

@@ -27,3 +27,12 @@ func Disable() (state State) {
 func Restore(state State) {
 	riscv.EnableInterrupts(uintptr(state))
 }
+
+// In returns whether the system is currently in an interrupt.
+func In() bool {
+	// There is one exception that has the value 0 (instruction address
+	// misaligned), but it's not very likely and even if it happens, it's not
+	// really something that can be recovered from. Therefore I think it's safe
+	// to ignore it. It's handled specially (in handleException).
+	return riscv.MCAUSE.Get() != 0
+}