scan-build-py: fix multiprocessing error

Recent versions of python3's multiprocessing module will blow up with
a Runtime error from this code, saying:

  An attempt has been made to start a new process before the
  current process has finished its bootstrapping phase

This is becuae the wrappers in bin/ are not using the  `__name__ == "__main__"`   idiom correctly.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D87051