Teach the NCG about the dereferencing and naming conventions to be
used when compiling for a DLLised world.  Some cleanups on the way
too.  The scheme is that

* All CLabels which are in different DLLs from the current module
  will, via the renamer, already be such that labelDynamic returns
  True for them.

* Redo the StixPrim/StixMacro stuff so that all references to symbols
  in the RTS are via CLabels.  That means that the usual labelDynamic
  story can be used.

* When a label is printed in PprMach, labelDynamic is consulted, to
  generate the __imp_ prefix if necessary.

* In MachCode.stmt2Instrs, selectively ask derefDLL to walk trees
  before code generation and insert deferencing code around other-DLL
  symbols.

* When generating Stix for SRTs, add 1 to other-DLL refs.

* When generating static closures, insert a zero word before
  the _closure label.