mn10300: export certain arch symbols required to build allmodconfig

Export kernel_thread() and empty_zero_page so that allmodconfig can be
built for MN10300.

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>