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 bar;

class S
{
    int i;
    final int foo()
    {
        return i;
    }
}

void main()
{
    auto s = new S;
    s.i = 42;
    auto dg = &s.foo;
    assert(dg() == 42);
}