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 tangotests.gc1;

void main()
{
    int[] arr;

    for (int i=0; i<100; ++i)
    {
        arr ~= new int[1000];
    }

    printf("arr.length = %u\n", arr.length);
}

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