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_debug4;

void main()
{
    char c = 'c';
    wchar wc = 'w';
    dchar dc = 'd';

    int* fail;
    *fail = 32;
}