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)

char[] get()
{
    return "hello";
}

void param(char[] s)
{
}

void refparam(ref char[] s)
{
}

void main()
{
    char[] dstr = get();
    param(dstr);
    refparam(dstr);
}