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)

extern(C) int printf(char*, ...);

class A
{
    int i;
    void f()
    {
        printf("A.f\n");
    }
}

class B : A
{
    long l;
    void f()
    {
        printf("B.f\n");
    }
}

void main()
{
    A a = new B;
    a.f();
}