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)

module staticvars;

int func()
{
    static int i;
    return i++;
}

void main()
{
    assert(func() == 0);
    assert(func() == 1);
}