1. Prevent deadlock by unlocking JDStatesMutex when calling back to the
controller to request a push of new symbols. (If JDStatesMutex is locked
then the push operation can't register the new symbols, and so can't
complete).
2. Record MachOPlatform runtime symbols during bootstrap and attach their
registration to the bootstrap-completion graph, similar to the way that
deferred allocation actions are handled. We can't register the symbols
the normal way during bootstrap since the symbol registration function is
itself in the process of being materialized.
3. Add dlsym testcases to exercise these fixes.
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