gdb: use intrusive list for step-over chain

The threads that need a step-over are currently linked using an
hand-written intrusive doubly-linked list, so that seems a very good
candidate for intrusive_list, convert it.

For this, we have a use case of appending a list to another one (in
start_step_over).  Based on the std::list and Boost APIs, add a splice
method.  However, only support splicing the other list at the end of the
`this` list, since that's all we need.

Add explicit default assignment operators to
reference_to_pointer_iterator, which are otherwise implicitly deleted.
This is needed because to define thread_step_over_list_safe_iterator, we
wrap reference_to_pointer_iterator inside a basic_safe_iterator, and
basic_safe_iterator needs to be able to copy-assign the wrapped
iterator.  The move-assignment operator is therefore not needed, only
the copy-assignment operator is.  But for completeness, add both.

Change-Id: I31b2ff67c7b78251314646b31887ef1dfebe510c