Add m68k support to checkstack.pl

Regular expression combination by Andres Schwab

Signed-off-by: <geert@linux-m68k.org>
Signed-off-by: Jörn Engel <joern@wohnheim.fh-wedel.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>