Terrible hack to restore CAF handling behaviour in GHCi (it's
currently broken).

The story used to be this: in newCAF(), if the CAF is in dynamically
loaded code, then we save the CAF's info ptr in a spare slot in the
closure, and add the CAF to the caf_list.  The GC will retain
everything on the caf_list.  At any point the CAFs can all be reverted
by replacing their info pointers from the saved copies.

CAFs need to be retained for GHCi because they might be required in a
future execution; an optimisation would be to avoid retaining the CAFs
if we're in "revert mode"; i.e. the CAFs are all going to be reverted
after execution anyway.  Also, this only applies to CAFs in compiled
code; CAFs in interpreted code are currently always retained.

Anyway, the old story is harder now that I removed the code that
checks whether a pointer is dynamically loaded or not (:-)).  Rather
than re-instate that code, I created a new version of newCAF
(newDynCAF), and arranged that the dynamic linker redirects any
references to newCAF to point to newDynCAF instead.  The result is
more efficient than before, and takes less code.