lib/libc/string/ffs*.c: fix problems introduced with D40730

Remove stale dependencies on the remaining old assembly
ffs*/fls* implementations.

Sponsored by: FreeBSD Foundation
Reported by: jlduran@gmail.com, jhb
Fixes: ee8b0c43 (D40730)
Approved by: jhb
Differential Revision: https://reviews.freebsd.org/D40966