Assorted fixes:

  * Stop being pessimistic about output register allocation
  * Start to handle function descriptors: compute target GPs and so on
  when doing indirect calls etc. Not there yet, though. For the moment,
  we try to use _indirect_ branches wherever possible, to stress test
  function descriptors.
  * FP divide-by-zero should work now
  * add varargs (it doesn't work, though)

At this point, all of SingleSource passes (modulo C++ tests that are due
to issues with the CFE, see note in the README.) Much of MultiSource also
passes although there's still a ton of bugs around. Something for me to
work on tomorrow, then. :)

llvm-svn: 20960