Make sparse checker work for userspace files - it normally gets -nostdinc
separately, so avoid having it for userspace files.  Also, add -D$(SUBARCH)
for multiarch hosts (i.e.  AMD64 with compatibility headers).

It works, the only problem is a bit of bogus warnings for system headers, but
they're not too many.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>