OSX: fix#57534 registering thread dtors while running thread dtors
r? @alexcrichton
- "fast" `thread_local` destructors get run even on the main thread
- "fast" `thread_local` dtors, can initialize other `thread_local`'s
One corner case where this fix doesn't work, is when a C++ `thread_local` triggers the initialization of a rust `thread_local`.
I did not add any std::thread specific flag to indicate that the thread is currently exiting, which would be checked before registering a new dtor (I didn't really know where to stick that). I think this does the trick tho!
Let me know if anything needs tweaking/fixing/etc.
resolves this for macos: https://github.com/rust-lang/rust/issues/28129
fixes: https://github.com/rust-lang/rust/issues/57534