Add the a dummy MMIX backend.

This commit adds the boilerplate code needed to get the new MMIX
backend to *compile*.  The backend itself can *not* actually compile
anything yet.
MMIX: Implement basic formal argument and return value lowering

This commit implements a very basic form of formal argument
and return value lowering.  It is enough to allow the following
empty function to compile without crashing llc:

  define void @foo() {
  entry:
    ret void
  }

The instruction printer still needs to be updated to produce
the correct instructions.
MMIX: Add basic assembly code printing

This commit adds support for printing the assembly code
of very basic inputs.  Namely the empty function now
compiles to:

  foo:                                    # @foo
  # BB#0:                                 # %entry
	  POP 0, 0