py: Add mp_pending_exception global variable, for VM soft interrupt.

This allows to implement KeyboardInterrupt on unix, and a much safer
ctrl-C in stmhal port.  First ctrl-C is a soft one, with hope that VM
will notice it; second ctrl-C is a hard one that kills anything (for
both unix and stmhal).

One needs to check for a pending exception in the VM only for jump
opcodes.  Others can't produce an infinite loop (infinite recursion is
caught by stack check).

py/qstrdefs.h

@@ -108,6 +108,7 @@ Q(GeneratorExit)
 Q(ImportError)
 Q(IndentationError)
 Q(IndexError)
+Q(KeyboardInterrupt)
 Q(KeyError)
 Q(LookupError)
 Q(MemoryError)