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 classes7;

class C
{
    int i=0;
    void f()
    {
        i=42;
    }
    void g()
    {
        f();
    }
}

void main()
{
    scope c = new C;
    c.g();
    assert(c.i == 42);
}