Bump clang's __FreeBSD_cc_version, to cope with r328816, which removed
-Wno-error=tautological-constant-compare again (this flag is now out of
-Wextra after upstream https://reviews.llvm.org/rL322901). Otherwise
the MK_SYSTEM_COMPILER logic will not build a cross-tools compiler.

Reported by: jpaetzel, tuexen, Stefan Hagen