byteorder: make swab.h include asm/swab.h like a regular header

Add swab.h to kbuild.asm and remove the individual entries from
each arch, mark as unifdef as some arches have some kernel-only
bits inside.

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