The exception for FreeBSD was added in 0afe9d1 ("freebsd packages:
Init at 13.1"), but it seems to have been erroneous, as e.g. ncurses
fails to build:

	x86_64-unknown-freebsd13-clang++  -o demo ../obj_s/demo.o -L../lib -lncurses++w -L../lib -lformw -lmenuw -lpanelw -lncursesw   -lutil     -DHAVE_CONFIG_H -DBUILDING_NCURSES_CXX -I../c++ -I. -I../include  -DNDEBUG -O2  -fPIC -DPIC
	x86_64-unknown-freebsd13-ld: error: undefined symbol: _Unwind_Resume
	>>> referenced by demo.cc
	>>>               ../obj_s/demo.o:(TestApplication::run())
	>>> referenced by demo.cc
	>>>               ../obj_s/demo.o:(_GLOBAL__sub_I_demo.cc)
	>>> referenced by demo.cc
	>>>               ../obj_s/demo.o:(NCursesUserItem<UserData>::NCursesUserItem(char const*, char const*, UserData const*))
	>>> referenced 46 more times
	clang-12: error: linker command failed with exit code 1 (use -v to see invocation)

This is fixed by undoing the change, adding -lunwind on FreeBSD.

FreeBSD doesn't use LLVM's cxxabi implementation, for backwards
compatibility reasons.  Software expects the libcxxrt API when
building on FreeBSD.  This fixes the build of
pkgsCross.x86_64-freebsd.boost.