Run TLS destructors for wasm32-wasip1-threads

[surban]

The target wasm32-wasip1-threads has support for pthreads and allows registration of TLS destructors.

For spawned threads, this registers Rust TLS destructors by creating a pthreads key with an attached destructor function. For the main thread, this registers an atexit handler to run the TLS destructors.

[rustbot]

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