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 typeinfo4;

void main()
{
    auto ti = typeid(void*);
    assert(ti.toString() == "void*");
    assert(ti.tsize() == size_t.sizeof);
    void* a = null;
    void* b = a + 1;
    assert(ti.compare(&a,&b) < 0);
}