arch: arc: threads: Comment clean-up

commit 780324b8 ("cleanup: rename fiber/task -> thread")
seems to be done by a script and in that particular case turned
menaingful sentence into nonsense. Alas, threads might be in all
four states.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Anas Nashif <anas.nashif@intel.com>