Fix x86_64 RTAI math `CFLAGS`

- `rtai-config --arch` output changes with v4.0
  - instead use `$host_cpu` from `AC_CANONICAL_HOST`

- add `-mpreferred-stack-boundary=4` to enable SSE instructions in
  kmodules on newer GCC versions
  - http://mail.rtai.org/pipermail/rtai/2013-December/026198.html