depend-cleanup.sh: cleanup stale ffs.S dependencies for arm and i386

Now that depend-cleanup.sh handles 32-bit dependencies a bit better, get
rid of the stale ffs.S ones, otherwise an incremental build on amd64
will fail with:

cc: error: no such file or directory: '/usr/src/lib/libc/i386/string/ffs.S'

Fixes: ee8b0c436d72