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)

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

void func()
{
    try
    {
        printf("try\n");
        return 0;
    }
    catch
    {
        printf("catch\n");
    }
    finally
    {
        printf("finally\n");
    }
    return 0;
}

void main()
{
    func();
}