Add compiler-rt's libFuzzer, not connected to buildworld yet.

For now, the libraries can be built and installed using:

cd /usr/src/lib/libclang_rt/fuzzer && \

make obj && \
make depend && \
make && \
sudo make install

cd /usr/src/lib/libclang_rt/fuzzer_no_main && \

make obj && \
make depend && \
make && \
sudo make install

See https://llvm.org/docs/LibFuzzer.html for more information.