Simplify call-out for sse2.

For sse2, we don't need to do the fldz/fstp dance when doing a
foreign function call.

A cross-compile is needed!

 * src/bootfiles/20e/boot-2014-04-cross-x86.lisp:
   * Cross-compile script to enable these changes.
 * src/compiler/x86/c-call.lisp:
   * Make the float result tn be xmm registers, not x87 fpu registers.
 * src/compiler/x86/sse2-c-call.lisp:
   * Simplify call-out not to use fldz/fstp except for the minimum
     required for the 32-bit ABI.
 * src/lisp/x86-assem.S
   * Comment out the fstp/fldz insts that aren't needed for SSE2.