mksysmap: update comment about __crc_*

Since commit 7b453719

 ("kbuild: link symbol CRCs at final link,
removing CONFIG_MODULE_REL_CRCS"), __crc_* symbols never become
absolute.

Keep ignoring __crc_*, but update the comment.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>