Resolve race condition in PluginMain registry
Resolve race condition in PluginMain registry

Summary:
Discussed offline. Most likely there is some race condition or library is initialized twice for some reason.
This will enable plugins to work in fbcode/.

Test Plan: make test in fbobjc on local machine as well as on devserver.

Reviewers: mathieubaudet

Reviewed By: mathieubaudet

Differential Revision: https://phabricator.fb.com/D1779498

Tasks: 5984521

Signature: t1:1779498:1421175193:0d80449485182924aec99886f6e5aecf8193e746

raphaelamorim/facebook-clang-plugins,wangwei1237/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,caijinyan/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,caijinyan/facebook-clang-plugins,raphaelamorim/facebook-clang-plugins,CJstar/facebook-clang-plugins,CJstar/facebook-clang-plugins,wangwei1237/facebook-clang-plugins,wangwei1237/facebook-clang-plugins,CJstar/facebook-clang-plugins,CJstar/facebook-clang-plugins,caijinyan/facebook-clang-plugins,caijinyan/facebook-clang-plugins,wangwei1237/facebook-clang-plugins