Extfl unboxing, f80vector ffi

  * small fixes for extflonum arithmetic: two functions were not jitted.
  * experimental extflonum unboxing
  * f80vector type for ffi (sorry, not documented yet)

I used the extfl flag and therefore did much less copypaste than previous time.

Unfortunately the extflonum unboxing is not stable and is disabled by default.
You can uncomment USE_EXTFLONUM_UNBOXING definition in jit.h to turn on extfl unboxing.