Add Linux-rt Kernel Option

[DrVektor]

There is no linux-rt (realtime kernel) option. Why? Maybe it can be. Thank you.

[Torxed]

We've simply missed that it moved out of AUR end of last year. We could add it in a PR :)

[codefiles]

There is also linux-rt-lts. I have been meaning to add them but have prioritized fixes over some of the stuff I want to add.