Crash in plumbum module when importing .so linked with gold

I am not sure it is boost.python's fault, since there are a lot of variables in the equation, but please take a look.

I prepared the repro case and gathered some info in this repo: https://github.com/mmatrosov/plumbum-crash-repro

In short, if you `import` an extension library in c++ which uses `boost.python`, compiles with `clang` and links with `gold`, there is a crash in this line https://github.com/tomerfiliba/plumbum/blob/master/plumbum/commands/processes.py#L286 during program shutdown. It does not happen if the extension is not loaded, and I've never experienced this crash even though plumbum is widely used in our scripts.

Start with the linked plumbum-crash-repro repository and reproduce the shutdown crash in plumbum/commands/processes.py around line 286. Compare importing the Boost.Python extension with leaving it unloaded under clang and the gold linker. Done means identifying and resolving the crash in the described configuration, with the reproduction confirming clean shutdown.

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