Simplify call_into_c more, putting more of the functionality into

call-out.

 * lisp/x86-assem.S
   * Rip out all of cruft dealing with float return values and the
     need for maintaining the FPU state.  Just call the foreign
     function and return, letting call-out deal with the result.
 * compiler/x86/sse2-c-call.lisp
   * Handle the different return values depending on whether we
     declared the function to return a float or not.