First cut at the x86_64 native code generator.  Lots of code is shared
with i386, but floating point uses SSE2.

This more or less works, the things I know that don't work are:

  - the floating-point primitives (sin, cos etc.) are missing
  - floating-point comparisons involving NaN are wrong
  - there's no PIC support yet

Also, I have a long list of small things to fix up to improve
performance.

I think the small memory model is assumed, for now.