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>

arch/arm/include/asm/byteorder.h

@@ -15,8 +15,6 @@
 #ifndef __ASM_ARM_BYTEORDER_H
 #define __ASM_ARM_BYTEORDER_H
 
-#include <asm/swab.h>
-
 #ifdef __ARMEB__
 #include <linux/byteorder/big_endian.h>
 #else