fix checkstack.pl arch detection

uname -m was leaving a newline in $arch, and not passing the tests.

Also, printing the unknown arch on failure is probably helpful.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>