Add double-double-float support for x86.

bootfiles/19c/boot-2006-06-1-cross-dd-x86.lisp:
o Cross-compile script to use.

compiler/x86/float.lisp
compiler/x86/parms.lisp
compiler/x86/type-vops.lisp
compiler/x86/vm.lisp:
o Add necessary vops and support for double-double-float.  Based on
  sparc port, with cargo-culting of complex-double-float stuff to make
  it work for double-double-float.