skip xtc sampling when threshold is zero

[mm65x]

fixes #1036.

make_sampler() activates XTC when xtc_probability > 0.0 but doesn't
check xtc_threshold. with the default threshold=0.0, all softmax
probabilities pass the threshold, so apply_xtc() masks everything
except the least probable token.

added xtc_threshold > 0.0 to the activation gate so threshold zero
disables XTC, consistent with how probability zero already does.