kernel: sanitize thread options

Some thread options were made architecture specific, i.e. K_ARC_DSP_IDX. We
have other architectures with similar functionality and we need to be
architecture agnostic at this level.

Changed the option to be more generic. The option is now called K_DSP_IDX.

Also remove multiple level of ifdefs and guards around those defines to
allow for documentation to be published and added a note about required
configs in the docstring.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>