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 bug27;

int func(int a, int b)
{
    if (a == b)
        return 0;
    else if (a < b)
        return -1;
    else
        return 1;
}

void main()
{
    int i = func(3,4);
    assert(i == -1);
}