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 mini.norun_debug9;

struct Foo
{
    int a,b,c;

    void func()
    {
        int* fail;
        *fail = 0;
    }
}

void main()
{
    Foo foo = Foo(1,10,73);
    foo.func();
}