kernel/freebsd: always use clang for kmod compilation

kernel/freebsd: always use clang for kmod compilation

Clang is the system compiler for FreeBSD and kernel module builds can fail
when built with gcc, e.g. when testing with test-meson-builds.sh.
Therefore, it's safer to always use clang to build the kmods since the
actual flags used are outside of DPDK's control and cannot be guaranteed to
work with all compilers.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>