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_debug1;

void main()
{
    int* ptr;

    // all these should be inspectable
    int i = 1;
    int j = 2;
    long k = 3;
    float l = 4.521;
    ubyte m = 5;

    *ptr = 0;//cast(int)(i+j+k+l+m);
}

extern(C) int printf(char*, ...);