`configure.in`:  `Module.symvers` handling tweaks

**MAINTAINERS**  this is a test!

- Remove `xenomai-kernel` `EXTRA_SYMBOLS` file
  - Obsoleted by dd99c60b, 'xeno_math: move to Submakefile'

- Check that RTAI `Module.symvers` file actually exists
  - If it doesn't, bomb out instead of creating unloadable modules