cross-tools: fts has different types for cmp routine

fts has different types for its compare rotuine. Other systems, not
4.4BSD based, have a non-const version. Before we tested against
GLIBC, but now we test against linux because that's Linux's API
and musl doesn't define GLIBC.

In addition, link against libftl on this platform since musl doesn't
include ftl routines in libc, but rather in libftl.

Co-authored-by: Val Packett <val@packett.cool>
Sponsored by: Netflix
Pull Request: https://github.com/freebsd/freebsd-src/pull/1066
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45349