Add just enough infrastructure to the NCG that it can deal with simple 64-bit
code on 32-bit platforms.  Main changes are:

* Addition of a simple 64-bit instruction selection fn iselExpr64 to MachCode.
  This generates code for a 64-bit value and places the results into two
  virtual registers, related thusly:

* Add a new type VRegUnique, which is used to label Stix virtual registers.
  This type used to be a plain Unique, but that forces the assumption that
  each Abstract-C level C temporary corresponds to exactly one Stix virtual
  register, which is untrue when the C temporary is 64-bit sized on a
  32-bit machine.  In the new scheme, the Unique for the C temporary can
  turn into two related VRegUniques, related by having the same embedded
  unique.

* Made a start on 'target metrics' by adding ncg_target_is_32bits to the
  end of Stix.lhs.

* Cleaned up numerous other gruesomenesses in the NCG which never came
  to light before now.   Got rid of MachMisc.sizeOf, which doesn't make
  sense in a 64-bit setting, and replaced it by calls to
  PrimRep.getPrimRepArrayElemSize, which, as far as I'm concerned, is the
  definitive answer to the questio `How Big Is This PrimRep Really?'

Result: on x86-linux, at least, you can now compile the Entire Prelude
with -fasm!  At this stage I cannot claim that the resulting code is
correct, but it's a start.