Bug 23756 – Add thread local constructor/destructor pragma

It is necessary to have a libc-based constructor/destructor at the thread level. This would replace DllMain for thread initialization and allow us to do TLS initialization in -betterC (although we may want to switch to it outright, for threads not registered with runtime). Some MSVC specific details: https://lallouslab.net/2017/05/30/using-cc-tls-callbacks-in-visual-studio-with-your-32-or-64bits-programs/

This is also handled at the object file level apparently. https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#tls-callback-functions There is no reason for SimpleDllMain to exist.

> It is necessary to have a libc-based constructor/destructor at the thread level. https://github.com/dlang/dmd/pull/14669

I had a look at that PR. Nothing in it looks to be adding a per-thread constructor/destructor. It only adjusts the existing crt_constructor/destructor behavior. Without changing what list of hooks it gets added to. From what I can tell it is unrelated to this ticket.

Glibc offers support for C++ thread local destruction as part of its implementation of pthread. https://sourceware.org/glibc/wiki/Destructor%20support%20for%20thread_local%20variables Via ``__cxa_thread_atexit ``. I don't know how we could hook into this, but it seems like we could do it if we wanted to.

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