message: Make the NULL backend proctables be generated in a .cpp
  Previously they were in a .mm and GCC didn't know what to do with it.
diff: @@ -429,7 +429,7 @@ def main():
  null_params = {
  'namespace': 'null',
  }
  - renders.append(FileRender('BackendProcTable.cpp', 'null/ProcTable.mm', base_backend_params + [null_params]))
  + renders.append(FileRender('BackendProcTable.cpp', 'null/ProcTable.cpp', base_backend_params + [null_params]))
  if 'wire' in targets:
  renders.append(FileRender('wire/WireCmd.h', 'wire/WireCmd_autogen.h', base_backend_params))
  