xtensa: Cleanup linker script using new linker script macros.

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Cc: Chris Zankel <chris@zankel.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>