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 condexp1;

void main()
{
    char[] a = "hello";
    char[] b = "world";
    int i = 42;
    {
    char[] c = i > 50 ? b : a;
    assert(c is a);
    }
}