Make checkstack work for UML.  We need to pass the underlying architecture
name, rather than "um" to checkstack.pl.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>