xtensa: Export __invalidate_icache_range

When modular code calls flush_icache_range():

    ERROR: "__invalidate_icache_range" [drivers/misc/lkdtm.ko] undefined!
    make[1]: *** [__modpost] Error 1

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>