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 bug54;

extern(C) size_t strlen(char*);

// creating args for main
void d_main_args(size_t n, char** args, ref char[][] res)
{
    assert(res.length == n);
    foreach(i,v; args[0..n])
    {
        res[i] = v[0 .. strlen(v)];
    }
}

void main()
{
}