The few platforms that cannot use the generic
get_signal_to_deliver implementation cannot do
so because they do special things for ptraced
children.  This can be easily avoided and thus
all of the signal handling code duplication can
be eliminated.

This is the first part, which adds a platform hook
right before the parent of the ptraced child is woken.
Data can be passed in via a cookie argument.

The next part will be dealing with platforms
that need to muck with breakpoints in the child
in this same code block.