Allow tearing down thread-allocated space from another thread

[jrvidal]

Motivation

👋 Hello again. It turns out that, in our use case, we don't fully control the workers so we don't always have the chance to cleanly tear down thread-specific space with __wbindgen_thread_destroy.

Proposed Solution

The bare minimum would be to expose __stack_alloc together with __tls_base and accept explicit arguments for those parameters in __wbindgen_thread_destroy. That way, a "leader" thread could perform the tear down (acting like a sort of scheduler).

Alternatives

Do nothing. This goes deeper and deeper into accommodating thread-related use cases without guarantees of stability and no clear prospects on how Rust+Wasm+threads should/could look like.

[jrvidal]

This time we went ahead and explored a potential solution: #3177