scripts/checkstack.pl: add support for nios2

Adjust checkstack.pl for the nios2 architecture.

Link: http://lkml.kernel.org/r/20170116113052.15034-1-tklauser@distanz.ch
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Cc: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>