byteorder: provide swabb.h generically in asm/byteorder.h

This is needed during the transition to the new byteorder headers as the
swabb.h functionality will be provided from asm/byteorder.h in the new
version.  To avoid breakage on arches still using the old implementation,
provide swabb.h from asm/byteorder.h as well.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>