local refs

[Stephen Weeks]

> Right; invokeRuntime's don't return arguments, because the invocation
> might switch threads, and we don't have anywhere to stash the return
> result until we get back to the thread that wanted it.  So, we store the
> return result in globals and fetch them with ffi's.  This, in turn, only
> works out in this case because we ensure that no other thread trashes the
> stored values by using atomicBegin/End.  So, maybe a better
> correspondence is:

Actually, GC_copyThread and GC_copyCurrentThread do not allow thread
switches to happen.  But, since any limit check does, the critical
sections are still necessary.