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 globals2;

template Bool(bool b)
{
    const bool Bool = b;
}

void main()
{
    assert(Bool!(true));
    assert(!Bool!(false));
}