sysutils/cbsd: Fix build on mips, mips64, powerpc64.

This port unconditionally links against libc++. This needs to be removed
to build on architectures that don't have libc++.

PR: 231789
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl>
Approved by: olevole@olevole.ru (maintainer), mat (mentor)
Sponsored by: IntegriCloud (hardware)
Differential Revision: https://reviews.freebsd.org/D17352