xtensa: use the declarations provided by <asm/sections.h>

Cleanups:
  - Include <asm/sections.h>,
  - Remove the (different) extern declarations,
  - Remove the no longer needed address-of ('&') operators,
  - Use %p to format pointer differences.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>