Add support for gcc-style "make bootstrap" which will build the stage
1, 2 and 3 compilers in the same build tree.

The idea is to simplify building a stage 2 or 3 compiler (now you only
need one build tree), and also speed up the build: there's no need to
compile the libraries more than once, because the stage 1 libraries
are used for stages 2 & 3.

In ghc/compiler, the objects and .hi files are now placed in
subdirectories: ghc/compiler/stage1 has the stage1 objects,
ghc/compiler/stage2 has the stage2 objects, etc.

The story from the top-level (fptools) is this:

  make all 	  works as before (i.e. builds ghc (stage 1),
		  libraries, etc.)

  make stage2     builds the stage 2 compiler
  make stage3     builds the stage 3 compiler

  make bootstrap  does 'make all' followed by 'make stage2'
  make bootstrap3 does 'make all' followed by 'make stage2; make stage3'

In ghc/compiler, the story is now:

  make all	     works as before (i.e. builds stage 1 only)

  make boot          generate build dirs and dependencies for stage 1
  make boot stage=N  generate build dirs and dependencies for stage N

  make stageN
  make stage=N       builds stage N compiler.  Run it in-place using
		    ./stageN/ghc-inplace.

I haven't decided what to do about 'make install' yet, and this still
needs documenting in the Building Guide.  Also, you still get the same
$(GhcHcOpts) for each stage.