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.real1;

extern(C)
{
real tanl(real x);
int printf(char*, ...);
}

void main()
{
    real ans = tanl(0.785398163398);
    printf("%llf\n", ans);
    assert(ans > 0.999 && ans < 1.001);
}