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)

struct bar {
    int bar;
}

void main() {
    bar Bar;
    with (Bar)
    {
        assert(Bar.bar == 0);
        void test()
        {
            bar ++;
        }
        test();
    }
    assert(Bar.bar == 1);
}