I guess you could patch Linux-libre with this https://rt.wiki.kernel.org/index.php/CONFIG_PREEMPT_RT_Patch
Here's how to compile it then https://trisquel.info/en/wiki/compiling-gnu-linux-libre-kernel
I guess you could patch Linux-libre with this https://rt.wiki.kernel.org/index.php/CONFIG_PREEMPT_RT_Patch
Here's how to compile it then https://trisquel.info/en/wiki/compiling-gnu-linux-libre-kernel