Add support for x86_64 arch. to 'make checkstack' (checkstack.pl).

Signed-off-by: Randy Dunlap <rddunlap@osdl.org>
Acked-by: Jrn Engel <joern@wohnheim.fh-wedel.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>