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 mini.norun_debug5;

void main()
{
    int i = 32;
    real r = 3.1415;
    real* p = &r;
    func(i,r,p);
}

void func(int i, real r, real* p)
{
    int* fail;
    *fail = 666;
}