nios2: Use IS_ENABLED instead of #ifdefs to check config symbols

Make the checking for div/mul/mulx instruction config symbols easier to
read by using IS_ENABLED instead of #ifdefs.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>