Teach cross-SF/CREF to dump/load in .nib, .txe, .dkp.

Teach cross-SF/CREF to dump/load in .nib, .txe, .dkp.

These correspond to .bin, .ext, .pkd.  LOAD-OPTION still loads from
.bin, .ext, .pkd (and .com) for libraries that are installed and are
not macro definitions of subsystems being cross-compiled.

This backwards suffix business is getting a little out of hand.
Possible partial alternatives:

- .xbin/.xext/.xpkd

- invent a static linker in the toolchain that can run on the cross
  host rather than using make.scm and disk-save in the cross target

- devise a portable fasload that sf/cref can use in the cross host

- use a machine-independent file format for .bin/.ext/.pkd