Provide a "pypy3"-plugin project and make the PyPy-plugin compile properly on FreeBSD. Because pypy_setup.py needs to be properly included within the plugin binary the corresponding binary linking step has been changed to include an "-m" flag because on FreeBSD the current LLVM linker (ld.lld) obviously has no default setting. Per default it is derived from the first object file. But in the binary linking step there is no such .o file. It needs to be included within the uwsgi binary because it is neede to build the plugins when using the uwsgi binary with the "--build-plugin" option.