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 typeinfo8;
extern(C) int printf(char*, ...);
struct S
{
    void func()
    {
    }
}

void main()
{
    S a;
    auto ti = typeid(typeof(&a.func));
    auto s = ti.toString;
    printf("%.*s\n", s.length, s.ptr);
    assert(s == "void delegate()");
}