ls: Make -, apply to -s as well as -l.

While here, remove a bogus comment about a gcc bug. The bug was in ls,
which used an incorrect format string, and in libc, which accepted it.

MFC after: 1 week
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D46067

(cherry picked from commit 647d4a8cafd2c9b291cab388191bc7fcfe12a66b)