Use `xeno_math` by default for RTAI math

Reported by Mick/Schooner/@ArcEye

The ShabbyX/RTAI 4.0 `rtai_math` kmodule misses the `ceil` and `floor`
functions.  I haven't investigated to see where they went.

Michael Haberler added `ceil` in commit 9898998 and `floor` has been there
since the `xeno_math` library initial commit.

This patch builds the `xeno_math` kmodule for all kernel thread
flavors, including RTAI, and configures runtime to load `xeno_math`
instead of `rtai_math` for `rtai-kernel` threads.