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 typeinfo11;

void main()
{
    int[4] a;
    TypeInfo ti;
    ti = typeid(typeof(a));
    assert(ti.next() is typeid(int));
    assert(ti.tsize() == 16);
}