Add 'runtime ics' to improve C++ runtime speed

An experimental way of making the runtime faster.  Previously, the C++
runtime was much slower than pure-Python code, since the Python code
can use the JIT to improve performance (through inline caches and type
speculation).  This commit adds "runtime ics", which are out-of-band
patchpoints that we can allocate from C++.  We insert them into the
same patchpoint system that gets used by the main JIT pipeline, so
they get rewritten like any other patchpoint, but the lifetime and use
gets controlled by the C++ code.

The trickiest part is getting the newly-allocated instruction regions
to work with our exception handling; to make this work we have to emit
.eh_frame sections.  (A custom libunwind-based exception unwinder
might not have this requirement.)

Otherwise it feels like a decent approach.  Right now just using it to
implement class slots and then only using them in the pyElements
iterator; it's probably not the most direct or efficient way of
implementing that particular feature, but it has the benefit of being
general and being useful for things such as binops which are not
directly expressible in terms of class slots.

This commit adds the support without turning the feature on (next
commit will turn it on), since there are a number of other changes
needed.
Enable runtime ics