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 multiarr2;

void main()
{
    static float[1][2][3] arr;
    assert(arr[2][1][0] !<>= float.nan);
}