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
```

py/qstrdefs.h

@@ -323,6 +323,10 @@ Q(mem_total)
 Q(mem_current)
 Q(mem_peak)
 
+#if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF && (MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE == 0)
+Q(alloc_emergency_exception_buf)
+#endif
+
 Q(<module>)
 Q(<lambda>)
 Q(<listcomp>)