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 bug51;
const ubyte[3] arr1 = 1;
const ubyte[3] arr2 = [1];
const ubyte[3] arr3 = [1:1];
void main()
{
    assert(arr1 == [cast(ubyte)1,1,1][]);
    assert(arr2 == [cast(ubyte)1,0,0][]);
    assert(arr3 == [cast(ubyte)0,1,0][]);
}