Only a few core funcs need to be implemented for SMP systems, so allow the
arches to override them while getting the rest for free.

At least, this is enough to allow the Blackfin SMP port to use things.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Cc: Arun Sharma <asharma@fb.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>