Run TLS destructors at process exit on all platforms

[surban]

This calls TLS destructors for the thread that initiates a process exit. This is done by registering a process-wide
exit callback.

Previously UNIX platforms other than Linux and Apple did not destruct TLS variables on the thread that initiated the process exit (either by returning from main or calling std::process::exit).

This also adds a test to verify this behavior.

@joboet Can we run CI tests for all available platforms for this?

r? joboet

[bors]

☔ The latest upstream changes (presumably #134108) made this pull request unmergeable. Please resolve the merge conflicts.

[joboet]

Sorry for the delay, I've been quite busy recently.

At a first glance, the changes look mostly good. There is a problem with the TLS destructor registration however: the TLS key can be used as soon as it is initialized, so it is possible that another thread completes before the destructor is registered. Always registering the destructor is not an option either, since that might lead to cycles in the destructor list. Windows uses INIT_ONCE synchronization to resolve this issue, but replicating that might prove difficult on platforms without a native Once-like interface (Once itself cannot be used in the TLS code since it needs TLS itself). I think this is best solved by keeping a thread-local destructor list similar to the one used on platforms with native TLS, but using the keys (or perhaps references to the LazyKeys) instead of pointers to values. Or perhaps you can think of another way?