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 tangotests.mem2;

void main()
{
    int* ip = new int;
    assert(*ip == 0);
    *ip = 4;
    assert(*ip == 4);
    delete ip;
    assert(ip is null);
}