Add support for storing args during an exception raised by an irq.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

unix/main.c

@@ -273,6 +273,9 @@ int main(int argc, char **argv) {
     char *heap = malloc(heap_size);
     gc_init(heap, heap + heap_size);
 #endif
+#if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
+    mp_init_emergency_exception_buf();
+#endif
 
     qstr_init();
     mp_init();