Fix cfloat return on x86_64: only perform ABI transformation for non-extern(D) functions. There's no need to waste cycles with extern(D), which we get to define ourselves. Fixes tests/mini/asm8.d. (Since the asm abiret code already assumed {xmm0, xmm1} returns)

bool got() { return true; }
extern(C) int printf(char*, ...);
void main()
{
  bool b = true && got();
  printf("%d\n", b ? 1 : 0);
}