1.0.15.14: move bignum allocation out of line on x86-64 from MOVE-FROM-[UN]SIGNED

1.0.15.14: move bignum allocation out of line on x86-64 from MOVE-FROM-[UN]SIGNED

 * Essentially port of the work done for x86 earlier (1.0.13.10.)
   Shrinks the core by about 500k.

 * Change the high-bits test on x86 to be clearer about what happens
   (no magic numbers.)

 * Fixups cannot be called directly on x86-64, so make the CALL
   instruction barf on that instead of silently generating bogus code.