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.atomic1;

pragma(intrinsic, "llvm.atomic.swap.i#.p0i#")
    T atomic_swap(T)(T* ptr, T val);

void main()
{
    int i = 42;
    int j = atomic_swap(&i, 43);
    assert(j == 42);
    assert(i == 43);
}