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.aa2_1;

int main()
{
    int[cdouble] x;
    cdouble d=22.0+0.0i;
    x[d] = 44;
    if(44 != x[d]){
            assert(0);
    }
    return 0;
}