nds32: linker script: GCOV kernel may refers data in __exit

This patch is used to fix nds32 allmodconfig/allyesconfig build error
because GCOV kernel embeds counters in the kernel for each line
and a part of that embed in __exit text. So we need to keep the
EXIT_TEXT and EXIT_DATA  if CONFIG_GCOV_KERNEL=y.

Link: https://lkml.org/lkml/2018/9/1/125


Signed-off-by: Greentime Hu <greentime@andestech.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>