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>
diff --git a/include/linux/swab.h b/include/linux/swab.h
index be5284d..ea0c02f 100644
--- a/include/linux/swab.h
+++ b/include/linux/swab.h
@@ -3,7 +3,7 @@
 
 #include <linux/types.h>
 #include <linux/compiler.h>
-#include <asm/byteorder.h>
+#include <asm/swab.h>
 
 /*
  * casts are necessary for constants, because we never know how for sure