[project @ 2003-10-22 11:11:59 by simonmar]

thread_stack(RET_BCO): Grab the pointer to the BCO *before* threading
it.  Fixes crashes when using compacting GC with GHCi.

I noticed while debugging this that compacting GC is horrendously slow
now, which might have something to do with the new eval/apply PAP
layouts.  That's something to investigate later.