New function: SB!KERNEL:%MULTIPLY-HIGH

New function: SB!KERNEL:%MULTIPLY-HIGH

 * Does the same thing as only returning the first value of %MULTIPLY,
   only better on some platforms.

 * Implemented vas VOPs on x86, x86-64 and PPC.  The PPC code sequence
   is fully untested, and merely looks correct.

 * VOPs for fixnum first argument are included, but will only be used
   when the result is forcibly marked as fixnum, e.g., with TRULY-THE.
   Questionnable, but I'd rather err on the side of straightforwardness
   rather than put even more pressure on representation selection.

 * Use it in the division-by-multiplication transform for unsigned
   TRUNCATE by constant.