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

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.