Support more general derived assembler labels
Write information about stack layout to Cmm

Idea is to pick it up later to generate DWARF stack frame information.
Reworked findGoodSourceTicks slightly

Going for the largest source span is just silly, really. Using the first
one we come across (while minding some priority rules) is less arbitrary.
Warning fix
Add "best" source tick and stack offset to debug records
Use Haskell name for naming DWARF procedures

Ideally, this would allow debuggers to refer to procedures by their
Haskell name. Somewhat unfortunately, this doesn't seem to be quite
enough yet.
Write DWARF stack unwinding information

We generate a .debug_frame section that tells (say) GDB how to unwind
stack frames. In theory this is easy: Given the PC, we get the stack
offset at the point in question, then essentially instruct to set

 Sp' = Sp - off
 IP = *Sp'

to unwind one frame. This works reasonably well apart from one major
annoyance documented in [Note: Info Offset]. Essentially, unwinding works,
but we have to cheat a little, and don't quite get all symbol names we
would want to see.